Because they historically didn't work on Linux. Looking at shooters from 2018:
- CoD BLOPS 4 - didn't work on Linux; it started working in 2022
- Battlefield V - doesn't work on Linux
- Far Cry 5 - unreliable according to ProtonDB
But taking it further, they're the gamer-iest games, so if you're playing one of these titles there's a high chance you're playing a lot of games, probably with friends, and each one your friend group picks up is another chance for Linux support to be poor, meaning that you're going to miss out. Obviously that doesn't apply to everyone, but it's absolutely going to reduce the number of people using Linux to play. With the Steam Deck now, this trend won't be as prevalent, especially for stuff played with controllers, but I bet you'll still see the phenomenon with AAA, multiplayer titles design for KB+M.