Comment by DiogenesKynikos

Because the journal is a stamp of approval, and scientists are measured almost exclusively through their publication record. Getting a paper accepted by Nature can make a career.

It's easy for anyone to publish papers online, but it's very difficult for a journal to build credibility and reputation. If you publish in some random journal no one has ever heard of, everyone will assume your paper couldn't get through peer review at a "real" journal.

That's why the established journals exist.