The companies you mention are not the only ones who require an NDA for product information and with good reason. I have had to work under NDA with TI (processors), Freescale (QorIQ series prior to full launch and now part of NXP), Vitesse (multi-gigabit crosspoint switch in the late 90s), Intel (networking devices), Broadcom (networking devices) and others.
A complex IC such as a networking device requires a lot of software support. The register map and detailed data on how to operate the device is part of the data sheet / reference manual. The functionality of the device at the deepest level (including specific pin functionality) is also part of the documentation. There is a great deal more, but this is an example.
If it were publicly available, a competitor could duplicate that map, internal functionality and pin functionality and make a device that is a drop-in replacement, (they would need to design a device which could be worth it for a large market) but that would reduce the selling price of the original part; this would make the investment required quite possibly exceed any return on the investment from sales.
Given the amount of money (several million dollars) that is invested in the design of such parts, it makes sense for the manufacturers to protect their product in such a way to protect their business model.
A market where a device is selling millions would be a very tempting target. Think SnapDragon parts used extensively in many designs. In the case of Broadcom, their advanced switching parts have some very advanced features that make them very attractive in networking gear as one example.
There are many parts that are sourced from multiple vendors and they are typically inexpensive for the good reason that they are available from numerous sources.
Note that the parts with a large market share would be tempting targets; this is the reason that a lot of data sheets are publicly available; the market is not large enough for a competitor to design a drop-in replacement.
Such things are standalone high performance ADCs (the market is growing but they do not sell in quantities of hundreds of thousands or more) and other high performance devices.
The ultimate reason is that the semiconductor vendors business model relies upon protecting their IP; for the ultimate in that arena, try getting your hands on the internal details of an Nvidia GPU; I worked with one (in a mission computer) and had access to the hardware manuals, but the software interfacing had to be done via a third party that works directly with them.
Each vendor has its own take on NDAs. Some may require a commitment to a minimum number of parts, others may not. I used Broadcom ethernet parts when I was at a startup that had the potential to use a lot of parts, although it ended up that we really did not use a huge amount.
The NDAs I have had varied widely; I even had a situation where a product line was bought up by a different company and parts that I had been able to get data for with no restrictions suddenly had to have an NDA; the opposite is also true - PLX required an NDA for a lot of their PCI express parts, but when they were bought by Avago (now part of Broadcom), the NDA requirement disappeared.
As noted in the comments, a lot of companies do not respond to emails from some domains (particularly educational, but in the past gmail, aol and the like were ignored).