Because the threat models are completely different.

For starters, if you get your taxes wrong, or if someone fraudulently tries to file on your behalf, it's a reversible process. You can fix it after-the-fact.

Ballots have to be anonymous (secret ballot) and also non-verifiable (to prevent vote-selling). There's no redoing an election later on, or asking a person to confirm their previous vote.

Someone might have an interest in impersonating you to steal your tax refund -- these scams do happen.

Impersonating a bunch of voters is potentially easier when you're just presenting yourself as a computer with an internet connection rather than a flesh-and-blood human body.

A single fraudulent vote doesn’t benefit anyone, but thousands of them benefit someone greatly, and it’s difficult to trace, as it looks the same as a regular vote.