They can. Each can pay the associated development and maintenance costs... & Apple, Google, steam etc demand their very reasonable 30% cut of each transaction. Credit card companies can charge their 4%.

The devs can even force users to keep all of the proceeds in their proprietary market and issue what are basically digital credit notes instead of real currency. Users only own their digital property in name only and can have their accounts closed, for numerous reasons, causing them to lose access to their digital purchases.

Do you think that is the optimal situation for both developers and users? If you can't see any room for improvement then you're probably right..... The current set up doesn't need blockchain.