Because nobody has ever been able to demonstrate to me, in a developed country, the mechanism by which an election is bought. Ads don't force people to vote. Nor do get-out-the-vote efforts. And the problem with special interests is not primarily about money, but term length and extent of government authority.
Money buys visibility. Visibility earns credibility. Credibility grants confidence. Confidence produces votes.
It's simple psychology. The more visibility candidates have, the more "serious" they appear as contenders. Voters then tend to vote for their favorite candidates among those who
appear to seriously have a chance to get elected.
In addition, greater visibility allows gives you an upper hand if you wish to discredit your competitors, because you can multiply your ads in much greater proportions than what they can retort to.
Obviously, visibility isn't *everything*, but it's pretty damn important.
As for "extent of government authority", sure, the more power the government has, the more interest there is in corrupting it. However, the less powerful it is, the less such corruption becomes necessary, because those who would do it can then easily do it other ways.
