Again, it's not about the company owning the data.
For instance, regulatory requirements might mean that someone comes to you and says...give me every interaction and related data that references this person who doesn't work here. That would mean anytime they are mentioned in word docs, records in Salesforce, anytime someone mentioned them in chat or email, and everything else.
I'm not saying that you aren't right. I'm just saying that the integration with other services allows for management at scale that is useful in large organizations.
Instead of going to lots of individual services and exporting data into something that you can then use to ad-hoc query, you can go to one place and create a job that goes out and collects the data for you. That has value.
In the end it gets back to the "best of breed" vs "pre-integrated"...It depends on how and where you assess value.
For some people, just not using MS has value. For lot's of people/orgs having all the things integrated already, even if they aren't the best tools available, is a better value.