because most home users only receive information; the only things that are sent are information requests and emails. You don't need killer bandwidth for that. And since bandwidth is a limited commodity (like anything else), it makes more financial sense for both the ISP and the home user to stick with a lower uplink speed. It's cheaper for both.
having said that, you can get Synchronous DSL (dunno about cable), but it'll cost you, and it's unlikely that as a home user you'd notice any difference. For the most part, only web servers and databases require faster uplink speeds.
though I suppose higher uplink speeds might be an advantage for real-time games, like Halo or Call Of Duty or something...