You only need a graphing calculator because you're not allowed to use wolfram alpha, desmos, or Matlab. Since you're mandated to use graphing calculators, (sometimes even specific models) there's no incentive to make them cheaper or better since you need to buy them anyway.
Yep, graphing calculators are a forced necessity for school, therefore they can charge anything they want and people will still buy them. This kind of artificial demand causes extreme price inelasticity and is capitalism at its worst.
Same deal with university books, you are forced to buy them so they cost hundreds of dollars, when they could easily be sold at a profit for a quarter of the cost.
But there aren't 'new' graphing calculators being required and they don't get worn out that easily. There's a relatively stable amount of people who need one at any given time, so honestly I'd have expected the second hand market to have crashed the market more than it has. There should honestly be multiple times over more graphing calculators in circulation than there is a need for them.
When I was in school, it was always specific models. They had to limit it to one brand and like 2-3 known good models to prevent the ones that could solve equations.
Schools have lists of approved models for standardized test taking. If students (the largest market) can't use your calculator, making one is probably not going to be profitable.