Because the other option is to support Trump
Being given that illusion, doesn’t mean that’s actually the case. There are other options, they just involve more than bare minimum action, including the discomfort of confronting and unlearning the indoctrination we’ve been put through from birth, and facing up to the fact that the system is rigged by design and is working as intended, and that no matter which of the teams you pick, neither is there to serve you or society, but the owning class, and themselves, and that only playing by the rules they have set out will never result in that changing.
As was the case here in the UK, it’s all they need to manufacture consent to maintain the status quo at best. In reality, the descent towards fascism continues no matter who is elected, and it will continue to do so as long as we play by the rules those in power have set out for us.