alt Hacker NewsI toured the Trek factory when they still made them in the US. They'd already drunk from the font of Goldratt wrt to Just in Time, but they would set up each day pretty much to make one model of bicycle for the whole day. Parts, tooling, paint booth, everything. The only thing that changed was sizes, and a model of bike tends to have the same geometries across all sizes. 78º angle here, 99º angle there. That may not be optimal for the rider but it's how you keep prices down and keep product lines from getting confusing.
If that's true of wheelchairs, you can get some economies of scale even if sizes vary. If it's not, then maybe that's one of the things we should tackle.
The bend angle of the tube that forms the seat support down to the legs seems like one of their major adjustment points for comfort and efficiency so I don't think you could have a similar setup. These are essentially semi-custom not a simple size based product like a bike. The extra adjustments are important because the users are in them many more hours a day so small problems can cause long term issues.
Their configurator has a very good model of what the chair will look like and you can see just how many knobs you can tweak and how that requires changing the core layout of the frame in a way that makes the kind of sizing system just not feasible. Scroll down on the Frame page to get to the fit sliders.
https://notawheelchair.com/pages/configurator
edit: Did the math and there's something like 25k different configurations they're selling before accounting for paint colors, just in the frame measurements. Granted, that's not accounting for the improbability or incompatibility of some parameter sets but that's still going to be a couple thousand different configs to build and stock. It doesn't work like a bike.