

The ELI5 version is that developers can make a lot of assumptions about what a Windows pc means and what features are available. A while ago if you had videos as part of a game (for example a cutscene) it was actually played through Windows Media Player, which was virtually guaranteed to be present on the user’s computer. Sure you can play that video with other tools like VLC or Quicktime, but you couldn’t guarantee they were installed, so Windows Media Player was a safe bet. Nowadays that’s not how video is handled but the point remains for a few other things. For example if I need to load an image, maybe a background, I would look it up using the windows filesystem, so probably something like C:\Program Files\Steam\common\mygame\images\background.png. That’s not the same in the Linux or another os. Also the piece of software that handles loading images might be different, which means how we execute that load operation is probably different, and so our Windows-focused version of our game just doesn’t work.
Fortunately nowadays that’s a mostly solved problem with Steam investing a lot of time into Proton, what they call a “compatibility layer” that basically translates all of the windows-specific stuff to work in Linux. That’s a very simplified explanation but you get the idea. The games that still won’t run have kernel-level anticheat (Valorant, Helldivers 2) or are so dependent on things only available on Windows that even Proton can’t fix it. Some anti-cheat software doesn’t run properly so then you can’t go online, like Warhammer: Vermintide 2. That’s mostly a commercial decision rather than technical, they could make it work they just choose not to.
Ok hear me out. I’m a book author, I write a book and try to sell it for £100 while all my peers are selling books at 60 or 70. I spend the most money imaginable making my book. It’s quite possibly the largest book in existence thanks to the effort of me and 5000 other people. I lie awake at night worrying that we’ll never make back the money we’ve spent on it.
Wait what’s this? Some team of less than 10 people has written a 3-page book and sold it for 2.50? And people are… Buying it?! But why? Look at the size of my book, clearly it must be better because it’s so big, so fancy, so expensive! Every letter cost me millions! I read the 3-page book. It doesn’t have money dropping from each letter like mine. It has a beginning, middle and end but mine has 500 acts each more expensive than the last. Surely it’s not that good… It’s pretty great actually. I have learned nothing from this experience, even though it’s happened a hundred times. I will still make more money than entire countries, somehow.