I would contend this is because many don't understand how to run their shops. As someone noted, they are often more artist than anything else.

I used to work in manufacturing, 24/7/365 except for a few down days here and there for equipment maintenance. An hour of unplanned downtime cost me $$$$ as we were booked to capacity 5 to 6 months in advance. I'm not suggesting that these shops can rely on many of the methods we used(we had much deeper pockets) but the inefficiencies are obvious and usually proprietor driven from my observation.