It’s a brand. They’re businessmen who rely, in part, on selling themselves so they standardize how they appear in public. They’re as much as a product as what they sell so making sure it’s all in brand is helpful.
This is why, in my opinion, Zuckerberg wore a suit to the hearings at congress. Not only because he needed to look formal but because he also needed to represent Facebook as more than just an extension of his supposed “genius”. He needed to show that Facebook was a serious company so he dressed as a serious person and not as “Mark Zuckerberg, the genius who’s so smart that taking a minute to pick clothes in the morning will make him stupid.”
I’m pretty sure Jobs just really liked those Issey turtlenecks and was someone who generally just devoted themselves to whatever they liked and just stayed the course.
Also, if you find something you like that much and have the means to buy it at that high price, why would you worry about whether there is something else with a modicum of the same style for a lower price?