Because that's how corruption and graft works in the US. Donating to political campaigns doesn't count as bribery. The donation is what gets them access to the candidate, so instead of writing a letter or calling their office you can have 30 seconds to a few minutes of their time at an election campaign fundraising dinner or over a phone call where they want to thank their donors during which you can press them to support whatever you personally want.

They're giving them money in exchange for the opportunity to try to convince the candidate to support what they want.