There are languages / runtimes where there could not be two different versions of the same thing in one binary (and they eagerly fail at build time / immediately crash upon run). That is not the case for JavaScript, Rust, etc. But it is the case for C++, Java, Go, Python and more.

Everyone claims different needs if they can. Nothing could be linked together anymore if you just let everyone use whatever they want.

Or maybe people start to try to workaround this by ... reinventing the wheels (and effectively forks and vendoring) to reduce their dependency graph.

There is a genuine need for single instance of every third party dependencies. It is not unique to monorepos. Monorepo (with corresponding batch change tooling) just make this feasible, so you don't hear about this concept for manyrepos, and mentally bind it to monorepo.