Would you want your brain surgery performed by a dentist; they are Doctors aren't they??
Most vehicles are a large investment. Maintaining ones vehicle in top shape is one of the best ways of protecting this investment. Detailing is like prevenitive maintenice. Along these lines, would you wait until your investment was belching white smoke before you gave it a tune-up and an oil change??
Car washes (most) deal in "quantity" and professional detailers deal in "quality" (most). The average tunnel wash does I think on the low end, 100 cars/day with a monthly employee turn around rate of about 68 percent. Low pay, hard work, a lot of overhead, and a lot of cars... where is the motivation for a quality product? This is not always the case, some car washes do a fantastic job and many times I take mine to one*. Another thing to consider is time efficiency... a Mobile detailer can come to you while you are at work, allowing you to multi-task.
These are just a few... I know there are even more.
*pm me if you need to know why