Speaking as someone who has laced motorcycle wheels (same construction as bycicle wheels, just with larger parts) it is very much a non-trivial task. Even given a premanufactured hub, rim, and a bag of spokes assembling a wheel is time consuming and easy to get wrong. Spokes have to be laced following the correct pattern, and it may not be obvious there is a mistake until ~80% of the spokes are installed. There are (at least on motorcycles with a drop-center rim) 4 different types of spokes that aren't that dissimilar from each other & are easily confused.

Having rewound electric motors w/ no repair manuals avaliable & laced rims w/ repair manuals & directions from the spoke manufacturer, rewinding an electric motor is far easier to successfully accomplish.

I've built bicycle wheels from parts. It's a bit tricky and time-consuming, but perfectly doable. The real tricky part IMO is getting all of the nipple adjustments just right so that roundness, tension, and track are all acceptable at the same time.