Different person here but I can give my own reason.

I stopped at the start of middle school. I was sorta just edgy while also hating he US military. More so to be a contrarian against my peers and specific adults, but that chip on my shoulder did come from me facing Islamaphobia growing up.

From there, becuase I was a stubborn little asshole (well I'm still pretty stubborn, but hopefully more reasonable) I would do it to spite people who asked me to do it or if I understood what it meant.

I don't think there was anything more annoying go my teenage self then somebody telling me:

"Just because the pledge/the national anthem/other 'patriotic' thing symbolizes something you see as bad doesn't mean others think those things have the same meaning. So you should do them out of respect for others."

Maybe my neurodivergent ass just doesn't get it, but that statement/sentiment infuriated me growing up, and I always found it insulting my intelligence. I came to be a leftist largely out of spite, contrararinism, and a longing to find flocks of other black sheep. I'm glad I started reading and enhanced my ideology into something actually coherent rather than stew in hatred and diverge into a more bitter person. A lot of those things still fuel some of my resolve, but I'm not guided by it like before. Ironically it was through materialism that I think I began to understand empathy.

Apologies for rambling but I didn't know how to finish my thoughts and just let myself ramble for the sake of it.