In webforms you could also render almost whole html by hand, except few tags like viewstate, eventvalidation and similar, which can be removed with PageAdapters. Nobody force you to use GridView or some other server side control that has bad html rendering output.
I would say that biggest advantage of MVC is SPEED!
Next is forced separation of concern. But it doesn't forbid you to put whole BL and DAL logic inside Controller/Action! It's just separation of view, which can be done also in webforms (MVP pattern for example). A lot of things that people mentions for mvc can be done in webforms, but with some additional effort.
Main difference is that request comes to controller, not view, and those two layers are separated, not connected via partial class like in webforms (aspx + code behind)