Probably some old experiments in there but the company had its fingers in a few pies and some departments didn't mind creating yet another service to solve a problem.
I definitely archived the old stuff in my department (we had eight repos and that felt like enough for three people).
When I left about a year ago, we had just started (after being on Github for almost 8 years) an ongoing project of first archiving old/outdated repos in place, and then moving them to an "archived" sub-org, and waiting to see if anyone complained.
Previously no one wanted to outright delete or remove repos because of the risk that someone somewhere was relying on it, and also there was no actual downside to just leaving them there (no cost savings, no imminent danger other than clutter, etc), so resources were never allocated to do it. There was always something more important to work on.
In an org with a higher floor of engineering management, a proactive program for removing unused or outdated repos would absolutely be expected though I think.
And it was each team owning multiple internal repos of their own deployments/libraries, and not, primarily, clones of public repos.
AI is making this even worse. With coding agents, anyone can throw together a quick internal prototype of any idea they have, even if it has no hope of ever making it to production.
The ones used for running the site itself.
Though, its so many that i think there are some customer ones in there too.
Some of those could be forks.