it's because they don't supply spare parts, or if they do, the prices, combined with the labor, make it not worth repairing.
But that is reality. It COSTS to have parts. To store, have inventory, to have additional overdraft, to have to stocktake, to have a complex inventory ordering system, to buy or sell a single part. In labour, in dead stock, that may never be sold. And in storage and shipping. So it really isnt viable or practical.
Thats why we have ready assembled sub systems like complete motors, complete joysticks, etc.
If you were to build a wheelchair or a car or any complex item from parts the cost will actually be 5 or 10x as much. And that is if the same level of profit is made.
Out in the comercial world, getting a tech guy trained enough to swap or rebuild things like motors or joysticks, costs, adds dangers, complexity, maybe voids legislation that a chair has to comply with, complicates warranties, and will result in bogging down the system. Then theres the stock and parts system complexity. The reason things are how they are is because a maker of things like complex electronics, be it a TV or a joystick, or a power supply, etc isnt geared up to supply parts. Its cheaper for THEM to just throw one away and make a full one. Its why in todays world we cant buy motor parts. Its not financially viable. These things are assembled for good - economically at speed. Bin it when it fails. Theres no drain plugs on many car gearboxes any longer. Theres no grease nipples on suspension or prop shafts. Everything today is manufactured on a fast cheap complex production line. Prop shaft wears out? Replace th whole thing it cheaper. The way things are made today not designed to be maintained.
As an END USER where reliability of repair, speed and ease of repair to a known standard doesent matter. We CAN take stuff aparrt, time isnt money. That 2nd callout doesen happen when the fix didnt work. We can take time, figure out how to fix it etc. Be we are not the target. We are unusual. Most chairs will just get a new joystick, new motor, order, fit, forget. By some service agent.