Because it's generally considered a sign of a very low budget movie if the names of the production crew also appear as actors.
Audiences tend to avoid very low budget movies
blcmitchell — 10 years ago(December 09, 2015 11:08 AM)
If you notice in the opening credits of The Beast of Yucca Flats, Larry Aten is credited three times with different variations of his first name. Surely for the sake of prestige, as the other posters say.