Why is there a bias against MP only games?

[SvennoJ]

I've bought Quake Arena and Unreal Tournament in the past. Just not my thing anymore, I simply skip them nowadays. Being up front about it is better than paying $60 for a game from which I'll never touch the multiplayer mode. I've overpayed for Halo 4, campaign was good, not $60 good. Actually I didn't overpay, traded it in, only paid $30 for the campaign which was worth it.