It is obvious if one bothers to actually look. But instead, you just wanted to hate on the US instead of spending even a minute looking and understanding. Try not letting your biases overwhelm your outlook in life and actually look at reality instead of jumping into assumptions.
The cheap products we've shared here just have a single list of specs for the dimensions. Here's an order form for one of those expensive chairs:
https://permobilwebcdn.azureedge.net/media/npxlfuoh/tr-tra-o...
Just look at how many different widths and lengths and angles and diameters these things are optioned out to. The different sizes, shapes, and styles of support inserts. And for a lot of those measures is not just that it is adjusted to those sizes; it's built to that size by order. There's not just a stock room with size 1, 2, and 3 chairs there; they're all custom made to order. Because everyone is different, with many in wheelchairs having very specific needs in terms of shape and support. It's like comparing a one-size-fits-all t-shirt to a custom-tailored suit and expecting it to be the same price, comfort, and fit.
And then on top of that all the components used in the higher end wheelchairs are just way nicer and far more rugged. Far better bearings. Higher quality fabrics and padding (most of those cheap ones are just plastic slings!). Wheels which are far truer and better built. Things which are welded instead of just cheap bolts and nuts holding it together.
Having a chair that doesn't fit you right or doesn't support you properly leads to more injuries. RSI injuries, pressure injuries, joint injuries, circulation issues, all kinds of other problems. You're bound to develop a whole host of issues if you're going to use one of these cheap chairs all day every day for your life.
It's like asking what's the difference between a wooden stool with a wobble and a $5,000 ergonomic rolling and reclining desk chair. Acting like those differences are non-obvious. Except, it's something the owner has to use every day of their life to go practically everywhere and do anything. For someone who has to be in the chair every day of their life, it is not just luxury. It is injury prevention. It is making their challenging life just a little bit smoother.
I'm certain whoever actually foots the bill for a nice, custom wheelchair wherever you are is probably spending similar-ish money for a similar-ish product. Maybe it is a private insurer, maybe it is the government. But if its someone actually needing to spend their life in a chair, I'd hope whoever is paying buys something far nicer than the cheap chairs we've linked here. They're not good for anything more than being wheeled around in a hospital for a short trip with an orderly pushing it. Or if you're just needing a chair for a few weeks to a month or two.
I do agree the US healthcare system is trash, but not because custom wheelchairs have a somewhat high cost. Its trash because so many end up needing to cover that high cost out of pocket. The people making a high-quality wheelchair from high-quality components should be well compensated in the same way someone making a really nice office chair should be well compensated. But since people need these to effectively function, cost shouldn't prevent people from accessing them IMO.