How did working for defense tech startups become cool?

Because war broke out. When there are no wars, everyone is against war tech. You could see it 5-6 years to 2-3 years ago people on here were loud about the fact that there should be no military. When wars exist, suddenly those same people are applying to Raytheon.

The reason to have a very strong military is because evil people exist. You can't let them win, so you have to be able to crush them if necessary. Often too, it means war won't even start if you have a strong stance.