In my experience and close friends, the agencies hire consultants to do the difficult work, and are not good at managing that more difficult work.

Agencies have no understanding of how to estimate a software project, and arbitrage underpaying moderately skilled workers who don't know any better working 50-60 hour weeks to compensate for bad schedules and lowballed estimates. I've never worked at an agency where a 40 hour week was remotely the norm.