Microsoft: "We, only one of most technologically advanced companies with only the 2nd or 3rd highest market cap of any public company on the planet depending on the day, had a problem with infrastructure trying to manage possibly the largest software project that anyone has ever made. And then we solved it."
You: "Stop doing what you're doing and pay attention to meeeeeeeeeeee."
So there is no reason to fix their mistake of a code base.
If no company is using many small repos for truly massive projects, then it's hard to argue it would be a good idea. Could everyone who has looked at this problem make the wrong choice?
Amazon. They built an entire system around managing versions so that they can make it work.
However, when you see that a large company is doing something in a way that you thing is silly or strange, the logical thing to ask first is "what do they understand that I don't understand?". It won't always be the case, of course, but most of the time it will turn out you were missing something.
Assuming off the bat that they are idiots and you know a better way is staggeringly naive.
There's even a name for this: "Going from D to C - from disparagement to curiosity". I think I first heard it from @patio11.