There are quite a few other routes in to these roles (start as a graduate, lateral hire from consulting/industry, internal moves, etc) and I don't think they really see stiffer competition or higher calibre candidates than you'd see for a front office role at GS or another bulge bracket IB.

However if you start at GS/another IB then you get plugged into a network where you see a lot more of these opportunities without having to go out to look for them as much. It's partially a numbers game so the more opportunities you see the better the chance of landing one.