Because ballots are cast in secret, but humans are relatively easy to influence, in almost every state, nobody is allowed to offer any kind of payment to people for voting or even "having voted". The more public the offer is, the more likely authorities will be pressured to prosecute the offerer(s).
How anyone actually votes is (supposed to be) secret. So your "Here's $$$ because I like the way you vote/ voted." scenario can be gamed by any dishonest voter. It is also illegal in almost every state to make or accept such an offer.
The laws generaly prohibit giving people "anything of value" in consideration of voting. So bottled water, food, coupons for food or drink at another location, raffle tickets, currency are all forbidden if they take place adjacent to a polling place, or giving public "prizes" for having voted.
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