I measured a few laptop AC-to-DC adapters.
1E6 ohm between GND pin (goes into the laptop) and earth prong (into the wall).
Desktops are grounded directly to earth: 0 ohm.
Why this difference?
The diagram below is based on the one you provided in your duplicate of this question. Not only are duplicates not alloeed they are a very bad idea as exactly what has happened here occurs - information is spread over two questions.
Your diagram was not of a power supply but of a line filter and it was left for right swapped.
I have rearranged it here and changed it to represent a power supply.
AC mains is connected at left.
DC low voltage is supplied at right.
The two series capacitors are the "Y" noise suppression capacitors required by most regulatory authorities.
IF the green dotted ground connection is connected then the centre point of the two capacitors is at ground, as is intended by the designers.
IF the green dotted ground connection is NOT connected* then the centre point of the two capacitors is at mains/2 at an impedance set by the two capacitors.
If a 1 megohm resistor is connected as shown it provides an electrostatic "bleed" from the laptop low voltage ground to either mains gtround or mains/2. Either will discharge electrostatic charge.
If the 1 megohm resistor is replaced by a hard link then if the green dotted earth connection is absent* then the laptop ground will be connected via the capacitors to Vmains/2. The impedance is low enough to cause very unpleasant but technically not life threatening shocks. It is also entirely adequate as a means of semi-random;ly destroying equipment connected to the laptop.
*Mains ground may not be connected because a 2 wire power cord is used, or a 2 pin plug is used, or a 2 wire mains socket is used, or because Murphy wanted to have fun. Using the 1E6 resistor rather than a hard link tends to give the best of all worlds in most cases.
I have here a Dell laptop power adapter where I measure 0 ohms between the mains ground and the ground of the low voltage output.
And I also have an Acer adapter where there is 1 Mohm like you measure.
So it is not a "universal truth" that all laptop adapters are grounded via 1 Mohm.
Indeed all desktops are grounded via 0 ohms.
For desktops the 0 ohm makes sense, if for whatever reason the metal case becomes live a 1 Mohm series resistor will not make enough current flow to ground in order to protect you, the user, when you touch the case or anything connected to it. Nor will it trip the RCD or earth leakage circuit breaker.
Remeber: metal case so 0 ohm grounding is required !
Laptops are a different class of devices as there is no mains voltage present in the laptop itself. The power adapter has a plastic housing so it cannot become live so 0 ohms grounding is not needed for safety.
This leaves it up to the manufacturer to make the choice of not grounding the low voltage side (mains side uses a figure-8 type power connector) or ground via 0 ohms like in a desktop or ground via 1 Mohm.
My guess for the reason to ground via 1 Mohm instead of 0 ohm might have to do with ESD discharging. When you are electrically charged and touch a metal part of the laptop, the discharge would flow through the laptop's ground. When this ground is 0 ohms you might feel a "tingle". With the 1 Mohms the current will be limited so the discharge will still take place (and take slightly longer but still only a fraction of a second) but you would not feel it.