Because software engineers are being asked to perform political decisions.
They don't really have a choice in the matter.
"I was following orders" is not a justification for committing crimes, but "I don't want to" is not a justification for not following lawful orders.
If you have actual reason to believe removing "DEI" from a drop down list on a website is not a lawful order, I would love to hear it. But you don't, which is why you're talking about soldiers and war crimes and trying to frame this as "an enemy...domestic" and not talking about what it is - changing text on websites.