"Difficult" usually means "not economical".

If your average person who wants fast-fashion doesn't really understand the different between knitting and crotchet, then you can simply having a knitting machine produce whatever you want inexpensively. There's a huge enterprise involved in making clothing at scale involved factories and shipping that's planned out a year in advance, so even if a couple of engineering grad students could build a machine prototype, it'd still be years out to putting it in production - you have to be fairly confident the economics of building that machine into a big factory and churning out tens of thousand of items of clothing makes sense.