Why are they always listed through some sort of other recruiting company? I feel weird sending my resume out to some mystery company that I have no idea who it is or what they do. I enjoy working at medium sized companies that do client-server applications, but I feel like the jobs I find listed on these sites are so vague about what they do and who they are, I have no idea what I'm applying for. Is this how most people find jobs? I know finding a job can be tough, but do other programmers not really care what they work on and will just accept any job that they can get? It just seems so frustrating to me.

Because most companies will entertain the services of specialised recruitment companies to perform the first selection of candidates.
And most companies that don't don't post on those sites, rather relying on more traditional ways like posting on their own sites or in professional publications.

There's nothing wrong with MOST such recruitment companies. The majority are good and can be trusted (of course there are always some bad apples).

And yes, it's how the vast majority of jobs are found that aren't found by invitation or by being made aware of them by a friend already working at the company seeking new employees.