Existing appstores like the ones on iOS and Android mostly target casual use cases, mobile devices and on-device software. Not "buy once" experiences for work via software as a service. They also do not offer a unified experience. Two "text-to-speach" apps could have completely different user interfaces.

The web does not have good discovery and reputation management and also does not provide a unified interface. That is why market places like Booking.com, Amazon, Spotify etc have become so big.