Why the fuck do cross-platform gaming doesn't exist? I mean isn't the API developed for that objective?

Minecraft ;)

Balance issues, most often, in an FPS, a controller is worse than KBM.

There are quite a few games that run natively on windows and Linux (can't say anything about mac, but who uses these for gaming anyways?)

Rocket league and ffxiv has cross platform play, but your right, It's rare to have cross platform. I think it's just unfare in most games if you play using console aginst pc user, but I would love to see more ps4, xbox one cross platform games.

"The API"?
What do you even mean?

Fortnite (Xbox One, PS4, Switch, PC, mobile) ...
Minecraft (Xbox One, Switch, PC, mobile) ...
Rocket League (Xbox One, PS4, Switch, PC) ...
Call of Duty: Modern Warfare (Xbox One, PS4, PC) ...
Hearthstone (PC, mobile) ...
PUBG (Xbox One, PS4) ...
Street Fighter V (PS4, PC) ...
Wargroove (Xbox One, Switch, PC)
And you can check more from SSEGold.

@Jannick any player of any of those games can play with other player using distinct console/platform each?

For example... GTA V player on Xbox 360 can play with a GTA V player on PS3? Same for PS4, PC?