Okay, I know why. It's because when the Constitution was written, the only practical way to have votes was to have Congressmen show up in person and cast their votes. And so, in grand tradition, House and Senate rules maintain this practice.
But today, it's hardly necessary that a Senator actually be present in order to register a timely, secure vote. We could set up a teleconference, or any number of secured networks in order to ensure only the actual Congressional member voted.
Think of the advantages: rather than flying cross country every week, Senators could actually stay close to their constituents for blocks of time, meeting with them and discussing issues with them. Those on the campaign trail could still vote. They could spend more time with their families and less time on aircraft.
They would be able to do their job perhaps not surrounded by Washington lobbyists, at least not all the time.
There would be no excuses of "Oh, I wanted to vote on that bill, but I wasn't in DC." No surprise votes where key members have to drop everything to return to Washington.
I imagine many people wedded to the current Way Things Are would object. If people are in town, they can be grabbed for dinner-and-arm-twisting. People out of town can avoid all that.
Maybe I'm just bitter about all the Senators and Representatives having to skip YearlyKos to vote on FISA. But I think, especially with climate change and conservation issues coming to the fore, that the time has come to embrace telecommuting technology.
So why don't they?