Competing app stores would raise another problem though.

One that occurred with certifying agencies in the financial crisis of 2008.

In a world with competing app-stores, if a store does its scrutiny properly, and is seen as too strict, then the apps will flock to get listed in other less-strict app-stores. Thus all stores are incentivised to relax the scrutiny to retain apps for their own survival. Sometimes to the point of simply rubber-stamping their approvals on crappy stuff.

Thus the end-users lose the benefits of proper curation.