Economies of scale and lack of competition.

When you have all the components for a cheap bike made in 100s of thousands by ie Shimano, the price goes down dramatically. Wheelchairs? Unless there is 1 dominant manufacturer its not going to happen to smaller shops, and monopoly has its own issues.

When you have regulatory tape which require some steep price hikes to cover some specific aspect (which may not be that important), combined with above you get what you get.

I was recently wheelchair-bound for a month due to my recent paragliding accident (crutches now for at least 1 month more, overall an interesting experience of various limits lying everywhere you don't even realize until you are there), and can appreciate even basic wheelchair and its various functionalities. Its simpler than bike for sure, but its also foldable (at least mine) and relatively easy to pack into trunk of any decent car in a minute (not for me of course but accompanying person).