It is not illegal for corporations to donate to political campaigns. It's also not illegal for corporations and lobbyists to offer jobs, to politicians after they retire from congress. Howard dean once a politician is now a lobbyist.
See: https://act.represent.us/sign/the-problem/
Corruption in America is perfectly legal, and the 1% carry a much larger voice than the bottom 90%, if politicians did what voters wanted it'd be a different story, see the video on that link for evidence that politicians vote 0% of the time with what the people want, and follow a more normal curve with the elites/corporations.
Money shouldn't = speech. This gives certain people more access to free speech, tips the tables, makes the country an oligarchy not a republic or democracy.
Imagine if all federal offices gave each person running $1 million dollars. Media stations gave each person 30 minutes/month during primaries and general elections for ads. No donations were allowed from corporations, and nothing from an individual exceeding 1000.
Imagine if lobbying were completely illegal. The anti-corruption laws (see represent.us link above) would fix this. It's a bi-partisan movement to fix corruption on BOTH sides of the aisle.