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).
Okay? That's the risk you accept when you take credit cards. People return shit. It's a fact of retail.
The catch is that a large amount of people use credit cards, so most shops (at least in the US) have no choice but to eat the returns. But it should go without saying that valve isn't gonna go bankrupt because of it or even suffer at all. (Kinda related video on how credit cards make money: https://www.youtube.com/watch?v=OceYCEexDqQ the main part I wanted to mention on how the middlemen of the processing charges make the cost of accepting credit cards at the shop more expensive compared to debit cards.)
Some places don't accept credit cards and may have a machine for people to pay their transaction fees on their own. Prices are better for everyone else as a result.