If you say "I can take two checked bags, it's included in the fare", I'll probably only take one because I only need one for the duration of my trip. But I'll feel good knowing if I decide I see something awesome while on my vacation, I can take it home without too much care for penalty fees.
But if you say "Each checked bag is $50", people will pack everything up to and including an entire 1996 Toyota Corolla into their free carry-ons, exhaust the available space, and the airline has to say to the economy class late-boarding group 'everyone after this line has to check their bag anyway, but we won't charge you the $50."
As an added plus, the price awareness reduces the quality of the travel experience-- you end up asking "can I fit another souvenir in my little free bag", and often deciding against it, because it's insanity to spend $50 in bag fees for a $10 book.