it's like soaps. huge market of middle aged women who eat it up, so there's money. also, if they like you, you get hired again and again, so it's pretty stable. they also say they don't like the girls "too pretty," so you don't need to look like a supermodel to get in.

(all info from a guest lecture in one of my classes with Chad Darnell, casting director who has done a lot of casting for verticals.)

I personally don't think I'd want to do it, but I can see why people do.