Why don't manufacturers sell directly to customers?

I worked for a company that designed stuff for satellite tv. They bought a machine to make printed circuit boards, reasoning that they spent so much on printed circuits that they really should do it in house and keep those profits in the company. Well, the machine didn't work. The company found out why printed circuits cost as much as they did. I pointed out that we spent a lot of money on wires too, but we were not buying a copper mine because we don't know anything about that business. A person or company chooses a business, and that choice excludes all other businesses. They lose some profits that way, but they are also protected from a lot of stupid mistakes and unexpected costs. And that is why manufacturers often do not sell directly to customers.