I do not argue against the fact that multiplayer games have a longer potential playtime, however not all games have multiplayer so you just can't say that a $50 or $60 game will definitely have 100+ hours of entertainment value.
You are probably looking at the wrong games. Most people who aren't already into games, won't buy the 60 USD games. Casual games will catch them. Think Solitaire or Tetris, which lots of people have sunk more than 100 hours over the years into. Or even evil Farmville.
I think in a sense you've identified the issue too. For every dedicated player who will pay $60 and be disappointed when he beats the game in 50 hours, there are 99 or 999 who want a game that they don't have to invest as much in.
Players like me want to play on our own terms and come and go as they please. I used to play a lot of WoW but shutting down all my VMs and rebooting out of Linux to do so is now a big disincentive. I have tried to get into some PS3 games but the long mandatory updates the system forces you into just makes me hate it. Oh my god and the loading screens that plague the PS3 are just horrid.
(Sorry I'm not trying to complain, just trying to give you guys some ideas. :-)