The reason for this is 2 fold:
1. They directly stop you competing with their current candidate, who is probably the highest % cut.
2. They indirectly stop you being put forward by other recruiters when you say "oh another recruiter has put me forward" (which they haven't, but you have a verbal contract with).
So the number of jobs you think you're applying to is actually less .
If you suspect this is happening (and make sure genuinely it's not just a delay in the HR dept first by asking probing questions) call the company and ask them directly, state that the recruiter is being a little slow corresponding and you'd like to confirm the status of your application. If they haven't heard of you, terminate your verbal contract with the recruiter and send your CV directly to the company. This happened for my current job, and now I'm earning a substantial amount of money (think tech arch, top end salary).
My boss said during hiring "I don't know why they wouldn't pass on your CV, you were clearly the better candidate".