Because they are cheap. Super cheap. Rent a house for a few weeks, throw some people in it, hire a camera crew and some editors, make a season of a "TV show", pay the "winner" $100,000. Or just follow some people around and film it and pay them... whatever. What could that possibly cost? Two million dollars? A million? Less? Or pay the cast of Friends a million dollars PER EPISODE. Each. Or pay Sheen $2MM per episode. Or whatever. I don't claim to know the first thing about Hollywood but I can easily imagine that a whole season of reality TV costs as much as one, maybe two episodes of a good TV series.