In my experience as a recruiter, founders have a very easy time finding their first few hires as they are all from the friends and family plan - all known quantities to some degree, perhaps a 2nd degree connection at worst. Then those few hires are probably good for a few more referrals.

Business starts picking up, all employees are working at capacity, and suddenly the pipeline dries up. This is when founders (and engineering leaders) start hitting up recruiters from their past. It's also the point where the company might be on the radar of recruiters, so engineering leaders are also getting incoming calls soliciting their business.

Under some conditions they do. Many founders don't have extensive social networks to recruit from. A fast growing company needs to fill positions and sometime the only time efficient way is to work with a professional recruiter.