If a big company (e.g. Facebook) integrated their app store within their main app, there's automatically a massive userbase available - it could potentially even slightly streamline the install process for apps advertised via Facebook.

I suppose this would mean that Facebook app itself would have to be sideloaded, which would probably result in a hit they wouldn't want to take. For a while Amazon had two versions of their app on Android, one in Play store and a second one you could sideload that had their app store integrated. Based on the fact that AFAIK they now only have standalone app store for sideloading, I assume it wasn't a popular option