probably to spice up the plot. Either way, they are annoying asf
That is unless they are the ml...that's when the author makes the prince's likable while sometime the other prince's (ml's brother) will be a scheming piece of shit.
It makes it easier for both the readers and the author to shit on them for being a hypocrite of a bitch
Basically that
yeah
why are all the princes in villainess isekais so mf conceited oh my god