It would require a blog post for me to answer this in detail, but overall it’s due to the fact that monorepos trend toward specific stacks and runtimes. They aren’t often as flexible as they’d have you believe, so you might find a low level programmer isn’t able to operate as efficiently when operating in a primarily NodeJS-based Monorepo. Plus it makes it easier to separate parts of your business that are distinct, simplifying the complexity of security, maintenance, and compliance.

I don’t think one should have 100s of them, practically speaking less than 5 should be enough to model any business today. It can be freeing though, and allows developers to take advantage of the tools that best fit there needs so they can go back to getting shit done.