That makes no sense. Say I'm an open source project maintainer who doesn't want PR's in my repo. I have to continually log in to GitHub to check the PR tab and close all active PR's (or as others have pointed out, use a bot to do this, but that's beside the point: The discussion is about why this isn't a built-in feature.)
What value does Microsoft get out of the "data" generated by me having to continually log in and close PR's? "Yup, people who don't want PR's on github log in a lot to close them". Why is that valuable? We keep talking about engagement as a thing but nobody's explaining why it matters at all for MS. "Engaging" me by forcing me to open up the website to do mundane stuff doesn't move any of MS's needles. "Usage" goes up only because I have to keep doing this mundane shit.
Here's a VASTLY more likely reason why MS doesn't want to make it easy to disable PR's: Lock-in. Microsoft wants to encourage users to use GitHub for everything involved in software engineering, end-to-end. Because if you do so, leaving GitHub becomes a lot harder, because GitHub has your PR history, your issue history, and is hosting your wiki, etc etc. This is not the same thing as "engagement", it already has a term, and that term is "lock-in". (Apropos: I consider PR discussions to be indispensably valuable in finding out why some particular line of code looks like it does: Finding the PR that introduced it and looking at the discussion is a great way to find out the motivation of the original author.)
MS does not like users that purely use GitHub as a mirror and don't use any of the GH-specific features, because those users can trivially migrate their code to another hosting provider if MS ever decides to do something silly like charge them.
Engagement makes no sense whatsoever as a motivation to not let users disable PR's. Lock-in makes perfect sense.