I suspect a lot of it has to do with the type of companies that tend to hire 3rd party recruiters. Namely, startups, and typically early stage startups who don't have any in-house recruiting staff. There's nothing at all wrong with early stage startup companies, except that my experience seems to indicate they don't actually know what they're even looking for. That's in addition to the likelihood that they just haven't been around long enough to develop a structured hiring process.
I don't know. What I will say is that your incentives and a 3rd party recruiter's are not the same. They want to place you quickly and get the commission, regardless if it's the best company or the most competitive offer. You're right that they do want to keep you at least somewhat happy, because if they get a reputation for treating candidates badly, that can be fatal in their profession. But, the people they really want to keep happy are the ones writing the checks. And, in this situation, that ain't you. You are quite literally the product here.