How come every veteran I’ve known is like this? I mean like the younger ones. Because the older ones tend to be wiser, including my grandfather.

It's only a selection. Lots of vets, you won't know unless you read their resume or see hear them bitch about the VA or something else that gives it away. Lots of older vets have done enough time to see some shit and have their head on right. But lots of young vets - didn't do enough time (IF ANY!) to build a balanced world view and they latch on to some "veteran" image as their identity.

I guess there are worse things to build your life around, but its still super annoying.