They did/do this. One of the big problems is competing with customers. i.e. you go from being a platform (Windows, Unity, Shopify, etc) to an aggregator (Amazon, Spotify, Instagram, etc) and your relationship goes from the small store to Shopify. Similar to how you might browse Doordash for "food", rather than look specifically for "Chipotle".

Upside of an aggregator is discovery/distribution. Downside is you rely on the aggregator for that discovery/distribution.

Rare for billion dollar businesses to be built on aggregators, not so rare for them to be built on platforms.