Because Steam is fucking balls, throws up errors all the time, I can't even get offline mode running and sometimes I can't even play the game because Steam servers are down. I've said it before and I'll say it again, I shouldn't need to have to piss about with some external program just to play a fucking game, every other game I have is just straight install, HL2 takes bloody ages to setup right and then you've got to hope Steam doesn't decide to play up or else you'll be doing nothing. If I can find a patch that disables Steam I'll gladly play HL2 and it's mods, otherwise I can't be arsed at the moment.
Seriously, hardly anyone else has these complaints. Wouldn't it be smarter to try and fix things and keep rants to yourself rather than posting abuse on a forum.