Because voting has very limited choice and a winner takes all mechanism. Ob-fucking-viously the candidates should be better, but not voting won’t make that change. Trump elected will just make this worse.

Voting is harm reduction.

If you want to make things better and promote your own idealised society, get involved, donate to causes you consider to be up to your standards. But even then getting involved and convincing future candidates will be much harder if Trump is elected than if it’s Harris.