Some people lack the motivation to actually work FULL TIME from home.
One of the most irritating things I find about working from home is the stigma that you don't actually work. I make a full time wage from home, and I work hard to get it. Long hours, throughout the night, and as someone else has already mentioned, when I'm down for 40 hours p/w I actually work for those 40 hours. Another one that bugs me is when people think you can't possibly be working on a computer at home. I have no idea how that makes sense, as it's no different whatsoever than me driving to a building that's not my home, sitting down in a very similar chair, and logging on to a very similar computer.
I don't get paid if I go off to have a 15 minute break, and I don't get paid for sick days, holidays etc. It's a tough gig working from home, and although it's more challenging than any other "real world" job I've had, it definitely has it's benefits.