Unless we're talking about some sketchy gig work nonsense, the "agency" is a consultancy like any other. They are a legitimate employer with benefits, w2, etc. It's not like they're pimps or something!

Those devs aren't code monkeys and they get paid the same as anyone else working in this industry. In fact, I think a lot of the more ADHD type people on here would strongly prefer working on a new project every 6 months without needing to find a new employer every time. The contracts between the consultancy and client usually also include longer term support than the limited time the original dev spent on it.