If they curate quality and safety, it does provide value.
There are hundreds of flashlight apps, for example, and many of them hoover up as much data as they can.
I can't bring myself to try mobile games anymore, because the discovery mechanisms ensure that IAP skinner boxes are preferentially surfaced in the app stores.
On desktop platforms, the app stores offer less functionality and little in the way of confidence that an app is "official".
No, "quality and safety" is a fig leaf which the platform owners use to justify their rent-seeking behavior.