While I do wish my AirPods could get louder, they still can get pretty loud. I think this may be a hardware/settings issue with your phone OP.

Edit: I believe with newer iPhones/the new iOS, if you listen to music too loud or too often the phone will prompt you about it being too loud. Iirc the prompt made it seem like if I continued to listen “too loudly” the phone would automatically turn the music down, but I’m not 100% on that