30% of every single transaction is nothing to scoff at. There are other peripheral justifications, like being able to deliver more relevant information about the game to your players than a generic store can, but revenue is the main reason yes.
Steam takes 30% of all sales.
When you spend $80 on Assassin’s Creed game off Steam, $24 goes to Steam and $56 goes to Ubisoft.
When you spend that same $80 on that same game but from Ubisoft Connect, $80 goes to Ubisoft.
The same is also true for buying that game on Xbox vs. Ubisoft Connect.
And it’s why you see so many mobile games try to encourage you to use a web store instead of the iOS or Android stores for in-app purchases.
It’s not very difficult to see why every publisher wants their own launcher.
It’s not pricing, because they’re priced the same in every store. But in one store you make a lot less money than the other.