Well, I know that on Upwork there are a huge number of clients who won't consider agencies, and that there are a lot of freelancers hiding the fact that they're operating as agencies (as you say they are on TopTal) because they know clients don't want to be assigned a rando to work with.
And clients who go to TopTal presumably choose it for a reason--and the big point of differentiation is the vetting. So if you're selective about who you're working with, you're going to be even more unlikely to roll the dice on an agency.
If TopTal started allowing agencies, it would be like a flashing banner that said "Your chances of getting a qualified freelancer are no better here than anywhere else," and they've built their whole business model on claiming those chances ARE better.