So pretty much every phone charger, tablet charger outputs 5 volts.
5 volts was part of the original USB specification which I assume had some influence on this, resulting in it just being easier to design things to be powered by such a common power source.
However is there any good reason why 5 volts was chosen?
While the original call for 5V was doubtlessly for TTL (as mentioned in the comments, specifically the reverse-biased BE junctions of bipolar transistors which are almost ubiquitously rated for 6V), there are several other things that play nice with the 5V standard:
On the last point though, I'm not really sure what's the causality direction. It could be that Li-Ion fitted perfectly with power supply rails for typical small electronics, which led to its ubiquitous use.
One of the reasons was that the European Commission facilitated an agreement among major handset manufacturers to adopt a common charger on the basis of the micro-USB connector for data-enabled mobile phones sold in the EU.
And USB specifies 5V.
One mobile phone charger for all campaign
Background:
In the past, mobile telephones were only compatible with specific mobile telephone chargers. An estimated 500 million mobile phones were in use on 2009 in all EU countries. The chargers used often varied according to the manufacturer and model; and more than 30 different types of charger were on the market. Apart from causing inconvenience to the consumer, this created unnecessary electronic waste.Almost every household is believed to have gathered a number of old chargers – estimated to generate more than 51 000 tons of electronic waste per year in the EU.
What the Commission is doing
In response, the European Commission facilitated an agreement among major handset manufacturers to adopt a common charger for data-enabled mobile phones sold in the EU.In June 2009, a Memorandum of Understanding (MoU) was signed in which mobile phone manufacturers agreed to harmonise chargers for new models of data-enabled handsets, coming onto the market as of 2011.
As a result, Europe's major mobile phone manufacturers agreed to adopt a universal charger for data-enabled mobile phones sold in the EU. The MoU committed the industry to provide charger compatibility on the basis of the micro-USB connector.
I have a strong suspicion that it's related to the typical lead acid battery voltage of 2.1V/cell.
Back in the day, lead acid cells (rechargeable) were a convenient way to power vacuum tube heaters (which took a fairly high current) while dry cells (primary) were adequate for the anode supply.
So most vacuum tubes used increments ot 2V as a heater supply - 2V for portable wireless sets, 4V was the UK standard pre-war, while "6V" more precisely 6.3V was a transatlantic standard and pretty much took over during/after WW2, around the time large vacuum tube equipment (radars, early computers, etc) started being made.
Consequently, entering the 1960s, most high current AC transformers produced ... 6.3V AC.
Multiply by sqrt(2) for the peak voltage, subtract a couple of diode drops for a bridge rectifier, another volt for capacitor ripple, a volt or so for headroom in a linear regulator and you're left with ...
yup, 5V would be about the nearest round number.
This is a plausible rationale for picking 5V in he early days of (not TTL but something earlier, maybe RTL) IC logic. I have no documentary proof that this is the case; evidence either way would be welcome.
However sometime in the 1960s it DID become established as the "normal" logic voltage and everything else (unless it had a compelling reason, like ECL with -5.2V) was made compatible with that.
(Until process shrinks made 5V/gate width an unreasonably large number in volts/metre, when voltage rails started reducing)
(I can't trace it back to the width of a horse's ass in Roman times, but perhaps somebody else can...)
PCs have been locked into 5 volts DC since the very early Intel 8008, because that's what TTL circuitry used, and the industry relied on TTL clear up until the IBM PC. As you know, IBM did not do a clean slate PC design, but made use of off-the-shelf tech, including lots and lots of TTL. So ISA cards were 5V, making it necessary for every PC that supported ISA cards. It's like railroad gauge; you can't change it because you need compatibility.
5V worked nicely for the objectives of USB, which included being able to build low-power devices that self-powered, including scanners, printers and hard drives. A lower voltage like 3.3V would have meant more current.
Understand that before smart phones, cell phone chargers carried only power. They were largely 2-pin barrel connectors of varying voltages (sometimes even AC!), with a lot of proprietary stuff out there. Every phone maker used a different charger with incompatible connections.
All of them were driving in the neighborhood of 2-5 watts. As it happens, USB was designed to allow 1-cable connection of many devices able to operate within a power budget: backlit keyboards, floppy drives, and inventive designers even found ways to make hard drives, scanners, printers, and CD drives work on this budget. This spec called out as 500mw for USB 2.0, and 900mw on USB 3.0. This is right in the range a phone is looking for to charge.
Of course, phone manufacturers allowed their own chargers to exceed 500ma.
Long before smart phones, there were "semi-smart" phones that could do things like take photos, carry address books and sync them to a PC, or have Franklin Planner type life-organizing features for scheduling and task management. All these things needed to sync to a PC. Bluetooth was not ready for prime-time (even today my FitBit can't talk to my computer, my 3yo phone or 2yo Kindle). WiFi is too complex and power hungry. Look at what wired ports were available on all 2005-era PCs/Macs (what? ethernet!?) -- the only choice was USB.
They didn't want to have two separate ports for charging and data sync, and why would they? As discussed, USB is right-sized for charging a phone.
It was a match made in heaven. And like apocryphal Roman chariot wheels defining railroad gauge, it all flowed from TTL.
So USB was already locked in for semi-smart devices. Then of course you had the Blackberry entering the scene. So USB was locked in as the connecting and therefore charging method, long before the iPhone 1 was designed.
Back in the early nineties, I was building control circuits around micro-controllers, specifically Microchip, Atmel, and Philips micro-controllers. USB was not even a thought back then, neither was Lithium battery technology. All the chips including the micro-controllers used +5V supply. I don't know about TTL (Transistor-Transistor Logic) but +5 volts is what supplied all the chips I worked with. If I am not mistaken they were CMOS (Complementary Metal Oxide Semiconductor) integrated circuits. CMOS draws less current than TTL, but maybe a little slower. I know that you can power CMOS with just 3 volts to maintain static memory, but for active circuits a little more voltage is required. The power pin for the ICs were often called Vcc (Voltage for Collector Current), which refers to the collector leg of the monolithic transistors that the ICs are composed of whether TTL or CMOS. So, +5 volts has been a standard for a long time.
