When you get a call from some shady recruiter asking you about a 6 month contract in Omaha, you're getting that call because a company or government agency decided to start doing strategic sourcing and procure people the same way they buy any other commodity.
So two things explain the poor quality -- they are literally squeezing every penny out of the process and are hiring idiots
The other thing is that they don't want to find candidates. They want to have low rates of success -- they're just engaging in a recruitment process to justify hiring workers on guest visas who can be more effectively exploited. If you look in an industry "trade rag", you'll usually find a page with some hard to read, small print job postings -- those are "compliance" advertisements purchased for the same purpose.
Even in situations where the employee profile is similar, recruiter quality can vary widely.