You have to think about scale too. A run of bike frames in a particular size, even for the high end, is probably four digits for a very niche customer. The cost of jigs and prototyping is well spread out.

For a given wheelchair, the market is probably an order of magnitude smaller, if not more so. That means that factory setup and development costs that amortize to $X00 for a bike produced in the thousands, balloons to $X,000 for a wheelchair.