I would say the reason they dont install controllers at the factory or the dealer is they have no way of knowing what kind of controller a buyer will want or need for their F150. Not all brake controllers are created equal. Not all controllers work well for all trailers or towing conditions. Some controllers work only with 2 axle trailers, some work with three or more axles. Some are proportional, some shut off when you brake going backwards, some people hardly ever tow and are happy with a $50 controller, some people tow alot or tow heavy and want a $140 controller, ect.
I think as you do some research on controllers, you will realize that your question is a little premature and naive.