Because as Trent Lott reminds us, the ones with the most money, win (buy) the prize...And the prize is the vote of the congressmen...
Without monetary donations, there is no way to fund political campaigns (unless you believe that the government should fun political campaigns). It takes dollars to buy airtime, print fliers etc. How is our system supposed to work if politicians have no way to raise money?
Furthermore, every American is a lobbyist. Send a letter, or a fax, or an e-mail and you are no different than the lobbyist that goes to the politician's office. He's just better at it than you are.