In my mind, a couple big reasons.
One, for the same reason that we accept a lot of awful things like murder as inevitable. Of course we'd rather get rid of these things, but it's unrealistic to think we'll get rid of them completely, so the systems we have in place for dealing with them when they happen will always be very important.
Two, because these things are often in the eye of the beholder. What I see as "rent seeking", you might see as "providing a public good". Most people (on HN at least) think current US copyright law represents a lot of rent seeking by Disney. But most of the same people still support some notion of copyright. There's no clear line where "public good that most people agree with" ends and "rent seeking" begins.
On a more serious note, there's not a lot that individuals can do about this – even individual politicians. The systems involved have to change pretty fundamentally to prevent lobbying from working. How do you think those systems can be changed to accomplish that?
I agree it's a hard problem, but I think with a big enough pool of information everyone could get a letter/email saying that politician x cost you y amount this year by voting this way. I know I would be compelled to make some changes.
Better than the status quo, since only huge companies will still be able to effectively lobby, but introduces problems of its own. That is, assuming the system works as intended. Which it won't.
Because how do you make sure the disclosure is mandatory? This punishes honest lobbyers (and honest lobbyees) more than it punishes dishonest ones; if all politicians are being lobbied evenly from all possible causes, but cause B's lobbying is secretive, politicians are incentivised to vote for cause B (i.e. against ¬B, which they are being publicly lobbied by).
This system does incentivise not being the ones to lobby, though – assuming there's no way to do lobbying that appears to be ¬B lobbying, so the politicians can be virtuous and vote B, which makes the “¬B lobbyists” lobby harder because obviously their bribes weren't big enough…