Skill level can be a factor. The more your skill level lets you be in tune with the process of what you are making, the more cost-effective your time will be.
Professional attitude. To sell things professionally, you need a certain attitude. That can involve things like understanding and identifying your market and exploring new ways to market your product.
If you are stitching as a way to cope with stress in your life, that can get in the way of thinking professionally.
I really enjoy stitching as a problemsolving methodology, so I would much rather teach people how to knit or crochet than make things to sell.
There is a balancing act between complete mass production and complete individual production. I know very few people who do much of anything truly 'from scratch.' I certainly don't grow the cotton that I crochet with, for example. As we have more choices every day for how we meet our needs, it can be a comfort to know that we can just go to a store and buy what we need. But it is also a wonderful thing that we can indulge in a process that connects us with the past, with the future, and with our environment.