Who are the people reviewing far more papers than they're submitting for review?
I can't imagine it's high-level professors, who seem constantly busy.


If you want something done, ask someone who's constantly busy.


  Nor do I expect it's early-career researchers, because they're unestablished and unlikely to be thought of when editors email reviewers.


I can't point to specific posts, but I've seen enough references on this site to the scenario that I believe it's not uncommon for the editor to ask the high-level professor and the professor to delegate to an early-career researcher in their department.