Beginners don't have that kind of money to spend on a 50 (especially one that isn't 1.4).
Who's to say only beginners use a 50mm prime ?
From my point of view there is naf all difference between f1.4 and 1.8 on a 50. I'd much rather have the slightly slower lens with a smaller objective lens and sharp fully open rather than a 1.4 that is really soft at that aperture.
If I'm out hiking in a beautiful landscape looking for pictures I don't want to be lumbered with carrying gear, and that includes a tripod, so IS on a standard or wide angle lens is very useful to me, and I'm sure to others.
50mm has traditionally been a lens that the manufacturer can offer obsolute top image quality with an affordable price tag. Not everybody wants to fork out for and lug around a lens such as the ( albeit fantastic ) 24-70 f2.8 II.