[throwaway5752]

"You'd expect that they would be incentivised to place you somewhere bad so that you're a repeat customer"

You answered your own question: "they're super expensive for the company who you hire through"

The person that pays the recruiter money is their customer. The item their customers pay for is the recruiter's product, which is you.

So the recruiter's reputation does matters to acquire additional inventory, but it matters more for their actual customers. Selling bad fits to their actual customer, the hiring companies, would be business-ending.

It is so important to understand when you are the product and when you are the customer.