Why do professors give talks on published work without their coauthors present at the seminars? They simply say that it is joint work with somebody.

Is it primarily because one is a better speaker than the other?  Like, do some professors publish a lot but choose to stay behind the scenes and let their coauthors present the work?
An incomplete list:


Because they're invited to give a talk, and their coauthor(s) aren't.
Because their coauthor(s) can't afford to travel to every talk they give on that piece of research.
Because their coauthor(s) are busy with other things.
Because this way they and their coauthor(s) can disseminate their ideas more widely, by giving more talks for the same overall investment of time.
Because the work is interdisciplinary, and the conference at which the work is being presented is devoted to some of the disciplines involved but not all of them, so attending would be irrelevant for the coauthor(s).


(Thanks to the commenters who suggested further additions.)