Non-US guy here. I don’t get why there’s a choice whether to vote for him or not, when you know the alternative is Trump.

It’s clear Biden isn’t a great option, and there’s probably plenty of reasons not to want him. But is any of those reasons gonna get better in the next 4 years when his opponent would win?

Not everyone wants to vote strategically. Giving a candidate your vote is a form of endorsement. If I was a US citizen, I’d vote for Biden, and try to convince all my friends to do so, too. But I also understand the people who can’t bring themselves to vote for a guy supporting a genocide, no matter what the alternative is.
They’re basically saying “fuck it, if the system won’t give me a choice to vote against genocide, then I’m not participating in that system.”