There's Stephen King. Not to mention John Grisham, Tom Clancy, Mary Higgins Clark. And there are lots of others. But some writers get big advances, their books don't live up to the expectations of the sales staff, and they get dropped by their publishers because they don't live up to the expectations of the sales staff. In other words, if you get a big contract, you have to produce big numbers.
Expectations are lower.
There are writers even now who are selling without agents. I sold my first two books without an agent. But an agent does help. It's just easier to get one after you've made a few sales. If I can do it, you can, too.