Because lobbies do have a legitimate purpose. Many lobbyists are extremely knowledgeable about the groups or industries they represent. Politicians are not very knowledgeable about all the various factions affected by the laws they write or vote on. The problem I have concerns specific instances where lobbies have swayed politicians or even written legislation that is not in the best interest of the public. The Enron Loophole is probably one of the best known recent instances of this type of activity. I think a lot of people are familiar with the name Jack Abramoff also which is another example.
I see your point, but even if lobbies are knowledgeable, aside from the occasionnal citizen groups and non-profit groups that want to support issues Xs and Ys, the mainstream lobbying is made by different parts of the industry, which means it's the industry, and not the citizens, who hold the largest leash on the politician's neck.
The point of lobbying is gaining influence by supporting the political campaign of key politicans with financial contribution, and make them.. supportive to your views. The lobbyist also use the "influencial way-in" to elected representative when they have an issue about legislation, governement contracts, etc... Because professionnal lobbyists just happen to know the Senator's main advisor, so your case will presented to him on a good way, since you paid well the professionnal.
It all end up with less power in the hand of the people, and more in the way of the wealthy. I don't care if someone is wealthy. I sure think less tax is good (but not at all cost), a good thing overall. I do not believe in punishing the wealthy people/industries for success. But I cannot stand in having a political system where wealthy can simply pay to have a quasi-direct influence over the executive/legislative body of a governement.
Having some dedicated NGO who listen the industries and support groups, forbidding commercial financing of any political-influencing organisation and the such would limit the cash flow into many publicist's pockets. And politican's. And it would also denying something I consider a destructive influence over the whole political climate in Washingtown.