Why are mono-repos a thing?

This is not necessarily a rust thing, but a programming thing, but as the title suggests, I am struggling to understand why mono repos are a thing.
By mono repos I mean that all the code for all the applications in one giant repository.
Now if you are saying that there might be a need to use the code from one application in another. And to that imo git-submodules are a better approach, right?

One of the most annoying thing I face is I have a laptop with i5 10th gen U skew cpu with 8 gbs of ram. And loading a giant mono repo is just hell on earth. Can I upgrade my laptop yes? But why it gets all my work done.

So why are mono-repos a thing.