Having never worked at a place the size of Google, I'm surprised that their IT team directly manages 100,000 machines. In my (apparently naive) mind, if you're smart enough to work at Google, you're smart enough to manage your own OS.

Is it a security thing? Are Googlers not allowed to directly control their own computers?

It’s for security and for homogeneity, a lot of google is setup around the principle that “works on my machine” is terrible, and also removing needless cleverness. You have root and can run anything you want*, but you have to go out of your way to configure anything differently than others, and the result is (hopefully) it just works the same everywhere. The monorepo also runs only natively on these Linux machines through a magical fuse interface so most development is either using the web ide or ssh-ing into the Linux box if you aren’t sitting in front of it. There were big economies of scale running this way and at least this setup, definitely felt pretty great and efficient I gotta say.

* By default, a tool called “Santa” keeps a naughty and nice list of runnable programs but all it took to get a program added to the nice list is any other googler vouching for it in an automated web tool.