Some musings from someone who has not worked in microsoft but has in big tech.

This often happens because the people inside are incentivized to build their own empire.

If someone comes and wants to get promoted/become an exec, there's a ceiling if they work under the an existing umberlla + dealing the politics of introducing a feature which requires dealing with an existing org.

So they build something new. And the next person does the same. And so you have 365, One, Live, .Net, etc

Google Plus was the same. Lots of unrelated google products were temporarily branded as part of google plus for some reason, including your google account and google hangouts (meet).