Economy

Political advertising is a huge chunk. Many campaign staff are paid. Campaign travel expenses are significant since the presidential and VP candidates are traveling often in separate parties to separate places. Rallies have to be rented with associated expenses .

None of this is free, and being cash strapped in a campaign can definitely make a difference in how many votes you get. But allowing infinite donations like this is not the only way to do things and ultimately grants the rich disproportionate influence.