Thats 50/month, but ok it's not for everyone.
Designing cloth is not something everyone can do and it takes practice and some are more talented at it so it makes sense to pay these people for their work.
More customisation is also harder to scale, if everyone start making their own color combination the knitting or weaving machines need to be re-threaded, which takes time.
Printing on fabric requires not just a printer but also an autoclave to cure the paint.
I'm actually in the middle of making a clothing CAD, and I would love to be able to make my own stuff but scaling so it's not 500€ for a tshirt.