Many no-compensation-listed jobs do not exist. They're there to hoover up CVs.
Anothrr reason, in the contract world would to maximize profits. Usually the agency makes their 20%, but it could increase drastically if they think you can be put in for less. I've seen them skimming up to a third off the top.
Company names are usually confidential because the agencies are all in competition with each other. Though a few do lock down sole agency agreements.
Often there are generic "xyz developer" roles that require 5 years experience etc... That they use from a previous client.
Remember recruitment agencies are sales funnels, and you're the product.
They're usually set up with a few account managers and a team of associates that work underneath them.
The associates rarely know anything about the skills involved in the role. They won't know the difference between MySQL and postres - just that they're different keywords. They could be straight out of school/university
Account managers are usually a little more savvy. These are the people who are in direct contact with the company hiring.
Often they'll get a decent database of clients and candidates then run off to another agency, or set themselves up as an agency themselves.
Some directors have been known to sell their agencies after building up a nice database of clients and CVs and be established for a number of years. Only to spring up again with that database after the non-compete clause in the sales agreement expires.
It's quite a seedy world out there in recruitment.
Always assume they're sharks and never reveal hiring manager names, or ex bosses. Some of them are really nice, but don't ever forget they're always after more information.