They tried it, returns and support cost them more. (There's a conspiracy theory about Microsoft bribing that doesn't make economic sense).
Linux users don't stick with things and often distro-hop. Linux is also modular. -They can get a Linux computer free of Windows by using their own principals: Build it from parts.
Linux users are generally anti-capitalists and have no moral compass when it comes to returning things or making prices go up for others. -Why cater to anti-consumers?
Which distro? -They all suck. An endless supply of distros makes it so it's difficult to figure this out (unless you can discern it from reading threads in a toxic Linux sub)
Linux software is volatile. A single programmer getting pissed off can stop development and take out multiple pieces of software depending on it because of its modular nature. (Imagine getting complaints about selling a computer with software that no longer works).
3rd party drivers and software can put excessive wear on components. Bad battery management can shorten uptime, create excessive heat and lead to short battery life for example, making decisions for OEMs complicated. Recently a driver caused screen flickering which could lead to a bricked display (and there's no warranty for Linux).
The majority of non-Linux games work just fine in Linux using Proton. The customer doesn't "know" until they are able to test it.
There are plenty of Windows games that don't work on Windows due to incompatible hardware, system requirements that are not met, or any number of other machine specific issues. Are those returns immoral also?
Let’s look at it from the opposite side: You order a washing machine from Japan knowing full well that the plug is incompatible with your socket. Is it justifiable request a refund because “it doesn’t work”?
Who gets a washing machine shipped from Japan just to return it? It's not like you're turning a profit from returning. This is just wasting time, same as buying a game for the express purpose of returning it. Nobody does that.
If the majority of Japanese washing machines work with an adapter that provides the correct power requirements (ie proton) then yes.