In real life too for example with most fighting champions they would not take a new coming fighter seriously especially when that person's body is not as trained as theirs, even if truthfully that person has the potential to train that much and get as strong as them or stronger than them, it's arrogance and ignoring others' value and potential, which may also reflect on them not valuing themselves much and so listening to celestial dragons or not training together more to get strong enough to easily defeat an emperor