The thought process here was pretty clear: YouTube probably gets a lot of referrals from 'Hey did you see this?' messages between friends, so why not let people do that directly from the site?
The first Google app to strike out on its own in the quest for a viable messaging solution was YouTube, which, in 2017, launched a feature called-what else- YouTube Messages! I see a lot of people who try to 'invent' this product as a joke, but it was a real thing that really existed for about two years, starting in 2017. This is basically 'The Google Docs Strategy.' They are very awkward since they are never compatible with anything else, and it's up to the users to manage a new, siloed list of contacts and message history. These usually aren't standalone apps but are instead messaging features that are glommed onto random Google apps. But, lacking a stable messaging platform from the rest of Google or a top-down directive to be compatible with whatever the latest client is, these service teams end up building their own bespoke messaging service. They realize messaging is important and look like they would like to plug into a single competitive Google messaging app. Looking back at this point in 2017, we start to see a series of decisions from popular Google apps that all seem to follow the same rationale. Google's inability to throw its weight behind a single messaging solution has a ripple effect across the company's whole ecosystem.
