* Store 1 offers them 70% of profit and no funding

* Store 2 then offers them 90% of profit and significant developement funding, on the condition of exclusivity

The existence of competition (including a potential counteroffer by store 1) gives the developer a better deal, and possibly results in a better game or ability to produce games that wouldn't otherwise exist.

It's admittedly a little bit of a hassle for consumers to use multiple stores, same as with games that have their own launcher/account system, but there's no buy-in like with streaming services or console platforms. I think it's probably worth it for a form of competition that has been lacking.