Mostly the machines the factory uses. You can cut tubes by hand and drill those holes, but the machines are more accurate and faster. However those machines will cost you in the million range each in some cases. (the $1000 Chinese versions for home use are kits that will cost you $10k to make accurate and the quality means they will wear out fast so while find for building a couple they are more expensive than the million dollar machines in the long run) That investment needs to be amortized across whatever you build and wheelchairs are not high volume.

You can't go with cheaper less flexible machines either because each wheelchair needs to be somewhat custom fit. That in turn means you need to the more expensive machines instead of simple jigs that. They also need someone to program the machines for your custom fit (or software to create that program)