Not everybody everybody. Some people want to charge/pay/receive the maximum reasonable amount. Where "reasonable" is informed by social norms. The existence of so many amoral corporations, and sociopathic individuals running them, has absolutely skewed social expectations though.
Such people are certainly less common, but they do exist (anecdata of one, me)
Homo economicus does not actually exist.
Tons of kids aren’t taught that, some of them start businesses, and they may struggle to make ends meet (or at least to thrive like they could be) because raising prices to market rates feels so unfair to them that they won’t do it unless prodded to and told it’s ok by someone else (and they still might not)
I definitely am not convinced market-rate-is-ethical-and-fair is natural thinking for most people, or the kind of thing they want to do.
(I’ve been the one telling people they should raise prices and I still can’t shake the feeling that it’s kinda wrong…)
Because it seems like the normal default for humans is that "fair" means when I sell something I take the highest offer I can get, but when I buy things the seller should give me the lowest offer he can while meeting his expenses or he is being "greedy." If you don't believe me just read the HN comments on any financial topic.