in those times everyone was conscripted , and people had a visceral feeling of fighting for their actual land and family out of necessity. Perhaps ukrainians have that feeling.
US army is more like mercenaries on a misson. Besides, Us soldiers have not fought on US mainland for century
Ukraine is far from a monolith. It's an agglomeration of bits and pieces attached in the aftermath of WW2 to a Ukrainian core. But there are plenty of ethnic Poles, Hungarians and Russians whose lands got attached that don't identify with it.
Before you downvote (OK, you can downvote first, I don't particularly care) - go look up what folks in Hungarian parts of Ukraine do to army recruiters.