I usually click 'fork' with the intention of making some minor change, and maybe committing it back.
90 percent of the time, that minor change either isn't suitable for being committed back to the project (it's badly hacked in, it's too specific to my usecase, etc.), Or the change ends up being harder to make or test than I had imagined, so I give up but don't bother deleting the fork.
That could explain the large number of forks and small number of PR's
