They exist and are popular because people don't want to think about the complexity.

Email was complex (and often not free!) before the popular webmails took over. Which essentially centralized the service to Gmail, Yahoo, and Hotmail/Outlook. (Do you not remember the awkward conversations where someone asked if they could message a Gmail from a Yahoo?)

The vast majority of users are nontechnical. They want it free and simple and someone else to take care of it for them.