Because we're taught to in school and everything we're meant to learn about enlightenment ideals and free, critical thinking is contradicted by our nonstop exposure to 'patriotic identity as a brand'. Our football games, our truck commercials, our news stations, everything is stamped with an American flag in the way a church might use iconography of crosses and saints. And since our education system is actually pretty bad at teaching nuance and critical thought, we latch onto it like "yeah, I like America, and I will glorify it because I'm being told to." And then you get into this really lazy binary of ideology as a commodity where people crave the dopamine hit of feeling like they're associated with a 'side', and a side which has all the most DeVaStAtInG hot takes to pwn the other side's strawmen. And then you're in "my country, do or die" territory because some wealthy person in the media is able to "rah rah" you into being a class traitor because of the trivial fandom associations they've taught you to make about culture war issues that actually have nothing to do with you or your day to day life.