Imagine there’s a huge fuel station that’s so big they essentially set rules towards suppliers that they can’t offer their gas to other stations at a cheaper price otherwise they can’t sell it in their station with 95% market share.
That is specifically not what is happening.
https://partner.steamgames.com/doc/features/keys
"Steam Key Rules and Guidelines"
"You should use Steam Keys to sell your game on other stores in a similar way to how you sell your game on Steam. It is important that you don’t give Steam customers a worse deal than Steam Key purchasers."
Valve does not "dictate" prices in any way.
Ubi and WB are free to sell their games for as cheap as they want. But if and ONLY IF they offer a steam key with the purchase, they can't undercut steam.