So there is no reason to fix their mistake of a code base.
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.
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?
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."