If you're asking why the games' options are like that, it could be so the console and PC versions can be as similar as possible. As people have mentioned, it makes more sense on a controller, with a thumbstick, to set both axes to different sensitivities. PC / mouse version probably just mirrors these config options.
Also I have to say I can't think of a game that does this off the top of my head, but when it comes to shooters I've mainly played older PC games, before the PS3 era. I think only setting sensitivity as a single value is common in those games.
I suppose the biggest difference I see is the imbalance between financial success / popularity, and discourse about many phone games. Compared to how many people apparently play some of those phone games, I never hear anyone talk about them. Because they're simply not worth discussing, not even to the people who play them.