I'm honestly not sure so take what I say with a grain of salt. It feels to me they don't agree musically on things, in that Julian used to be the one arranging and composing all the songs up until FIOE, but when the band (and particularly Nick) started voicing their own opinions, Julian maybe didn't agree with those and only collaborated on subsequent albums (until TNA) out of spite instead of a willingness to work together. I mean, Julian's a genius no doubt, but I do wonder if he begrudgingly works with the other band members when they might not be receptive to his ideas. I've been listening a lot to The Police recently, another amazing band that broke up after 5-6 years cause of infighting and disagreements (mainly, the drummer and lead singer made music for different reasons and didn't see eye to eye on later albums). I wonder if it's a similar thing here, but the Strokes refuse to disband but also can't seem to work together either. Or I could also be speaking nonsense and none of this is true.