Because in many situations there's a very fine line between "lobbying" and "making decisions based on the will expressed by constituents". An organized group of concerned citizens is a lobby.
When it comes to outright bribing politicians, that's easy (and illegal). But there's a ton of grey-area in there. In fact, I'm not sure how you even could remove lobbying from the political process.
So you get a group of interested citizens together, you write to politicians, you tell them your point of view. Nothing wrong here yet.
Then some of those politicians then agree with your group. You like that! So you start raising funds to get them re-elected. At face value, nothing here is wrong.
Except that you just used your money as an incentive for politicians to agree with you. They might now openly support you, not because they personally do, but because they want your financial support for re-election. Since actions speak louder than words, it just makes sense to support politicians who don't just say they agree with you, but to support politicians who actively enact policy you want.
At this point, we have reached 'paying politicians for enacting policy you want'. That is bribery. Where did it all go wrong? When your lobby was able to raise funds and spend that on getting a politician re-elected. And when a few lobbies being a 'concerned group of citizens' turned into a facade for corporate interests. Simply because there is just so much more money behind corporate interests than behind concerned citizens. And hence those lobbies get more influence.
How to solve this? Limit political spending to natural persons, and limit their contributions.
I think this would prove to be exceedingly difficult in practice. In fact, this kind of is the system we live under, and it doesn't work.
Sure, you could argue for changes in the amounts, and who can donate to what, but consider this: I believe Obama's biggest corporate donor in 2008 was Goldman Sachs, and the value of their contribution was on the order of $1,000,000 (correct me if I'm wrong). Considering the funds that Goldman Sachs has access to, and the cost of a Presidential campaign, this is a pittance.
Clearly there are other forces at work here. You can fiddle with the numbers, but I don't know how you set up a system that doesn't reward "cheaters" or the already wealthy (ie received their "contributions" in advance). Which is where we're at.
At this step:
> Then some of those politicians then agree with your group. You like that! So you start raising funds to get them re-elected. At face value, nothing here is wrong.
A relative of a friend was a politician. He used lobbyists indirectly to work on issues. Say he needed help understanding a school textbook issue. He would call up his oil lobbyist, whose job it was to make him happy. It was definitely not his job to piss him off. He would quickly get a report on the textbook issue, probably written by the expert in the field.
This is kind of a weak argument, because this goes for literally every law. Even though this is true, as a society you can still make the decision that it isn't okay. By making it as okay as it is right now, you're saying that it's just a fact of life and we need to accept it because of that. Murder is a fact of life too (as in: it happens regardless of it being illegal), but that doesn't mean we need to make it legal.
It's a wierd sort of politician problem solving, making do with the resources you have.
stop calling it "lobbying" like that means something special, it's called free speech and representative democracy. you don't like people speaking in secret? how about we register them, and call that lobbying.
irritated that the people who speak up get heard? that's called life.
you want to be heard, you need to speak up.