They didn't have a monopoly on information anytime in the 20th century. If you wanted to have all the knowledge, it could be expensive to acquire a personal library, but nothing was stopping you from acquiring the same textbooks. Heck, until the information age it was pretty easy to forge a credential.

But it wasn't about credentials even. It was about inculcating a culture. You knew that someone had the knowledge and ability to reach university, and you knew that they had a shared common culture with you. Shared common culture and norms increases trust. Credentials mattered for doctors, but universities, in the end, were selling something far more intangible: culture.