For those who are in organizations that have near 500 repos, why do many? Is it shear size of the company, or is it organizing individual internal libraries int

For those who are in organizations that have near 500 repos, why do many? Is it shear size of the company, or is it organizing individual internal libraries into different repos (seems like good practice), or something else? I've only ever worked for small companies where we have max 5 real repos, and I work at a company with a mono repo right now. Large scale systems are very interesting to me, but I have to say, I don't mind the mono repo.