i mean, sure, they can, but they will get angry emails from Valve
So? Maybe they should put more focus on improving their own platforms instead of whining about Steam, then they could sell their game exclusively there without issue.
I don't know I can make it any clearer; Steam has a big market share because people choose them, not because they use any hostile tactics to push out the others. Ubisoft and EA, among others, have for years done everything in their power to lose any good will from the player base in favor of their shareholders.