Because their friends play Fortnite, for example? Multiplayer is often social, so "just play something else" turns into "just get new friends".
i like a few multiplayer shooters, fortnite being one of them. i also exclusively use gnu/linux on my machines, so i got around this issue of games like fortnite/battlefield/etc issue a long time ago by simply doing what i've always done for years, playing these on xbox. i even 'stream' these games to my linux machine from my xbox if i want to play them from the computer with the xbox controller, and can join and create xbox live parties through the xbox web interface.
i only do this for those couple of games i play with friends that won't support linux because of the aforementioned rootkit it wants to run on windows machines. console for those games, and all my other games run happily either through steam+proton or natively on linux, and there are a fair few FOSS games with amazing multiplayer. i love luanti, xonotic, openarena, veloren, etc, and play them frequently with some friends. :)
There's another way. Only a small portion of friends need to change to pull the rest of the group. Pull them to a game that runs on Linux.
Don't do it like "let's play this game because it runs on Linux" do it like "let's play this game because it's fun".
If you want to be the one to lead this change you have to do extra work. Dual boot Linux and find a game that's fun that you can do online. Find the other friend or two in your group that will do the same (at least play the game, Linux is optional but encouraged for this subset). Just play together for a bit, give it a trial run. Then when playing the other game with the larger group say "hey, so and so and I have been playing this game, you guys should play with us sometime". They don't have to install Linux, just play a new game that their friends are already playing. That's why they're there, to play games with their friends. Don't try to get them to switch to Linux, just play games with your friends. You might have a holdout but if most people move then everyone will. But if you want to do that move you have to find what works and at least one other friend to give it a trial (who won't need to do as much work as you). That's how you do it. No crazy scheme and honestly not massive amounts of work either. Just the normal process of finding new games to play with one constraint. It just seems complicated because I stated the process explicitly.