Whether or not it's vanity, there's an element of enjoying being in the public eye--at least in some contexts. If someone really doesn't like being publicly visible they're mostly going to self-select out of being a speaker unless they have some other compelling motivation for doing so.

Whether or not it comes naturally or it's a persona they've developed for professional reasons, most public speakers like the energy of the crowd or other audience. (Personally, I'm much more comfortable live than on video--which is a somewhat different skill.)