Why is voting software so hard?

I understand there is A LOT at stake when writing software for voting, but how is it so different than writing software for banking? There is probably way more interest in stealing someone's money than their vote, but we use banking software on a daily basis and I can't ever remember a time when people just lost all of their money and and they couldn't figure out how to put it back? Everyone involved in the banking software industry has a strong motivation to account for each and every penny, why can't we do the same with voting software?