Other Benefits Not Mentioned Above

Easy single view of all open PRs
Every project will use same workflow (can't having diverging branching models across projects)
Have to setup IDEs less often (no need to setup Intellij 10x for 10 different repos for example)
Code sharing/reuse is discouraged in multi-repo due to the overhead. Suppose module A and B would naturally both depend on C, but instead they each have their own implementation, say C_A and C_B. Now if a fix/feature is applied to one, say C_A, it won't get propogated to C_B, or we have to fix the bug twice. In large projects this gets much worse.
Multirepo is an almost irreversible decision, it's hard to go from a multi to a mono, but it's easy to go from a mono to a multi.

Sorting out the gitlab creds plugin in multi-repo has to be done N times, instead of 1.

Upgrading any project to a newer version of a lib has to be done N times, instead of 1.

Divergent codebases due to mismatching libs.

Setting up gitlab settings, like branch protection rules has to be done N times, instead of 1.

Forcing Checks

All projects can be forced to conformed to same checks, like security checks, file size checks, etc.