There are a variety of reasons.

Some people feel that voting is offering material support to a specific candidate or system, and they cannot bring themselves to do so given the horrors that that person or system is either supporting or failing to condemn.

Others may feel that strategically withholding their vote as a punishment may motivate democrats to take these types of issues more seriously in the future.

Or they may feel that their vote is more impactful magnifying the voice and power of third parties who offer more meaningful solutions to end the killing, even if they won’t win.

Others still may believe that Trump’s incompetence will accelerate the end of America imperialism and lead to a better global political situation sometime in the future.

Finally, some people feel that voting won’t matter at all and is a distraction from efforts to directly slow or stop the war machine.

I don’t personally endorse any of these viewpoints, but some are relatively serious positions and others are not, in my opinion.