Google is a search company. Owning the platform is Google’s way of making sure they own search. Google does not care about facilitating app sales because they can make 15-30x the money from search.
Google benefits from owning the platform because the Google suite of software (AppStore, Browser, Mail, Maps), bundled on almost every Android device, drives search traffic to Google and encourages users to stay within the Google ecosystem.
Most Android OEM's are effectively forced to carry this software (and meet 'compatibility requirements' such as Google being the default search engine) because without it their devices are far less compelling. Replacing an email client is a solvable problem, a maps alternative is something that could be licensed, but creating an entire AppStore replacements is unrealistic for most companies.
Unless you are Amazon.
If the Amazon AppStore becomes a major player in the Android app market, and OEM's choose to license it, thus forgoing the Google software suite (most of which is much easier to replace), Google will be in serious risk of losing their leverage over OEM's. They will not be able to ensure Android devices drive users to their services, and could even end up paying OEM's (a'la Mozilla) to have Google as the default search option on these devices.