Why not go to college first?

I realize that I can do it after, or both at the same time. But going to college first will make you an officer and I'm assuming the money i spend will be somehow given back to me after my service on top of the extra pay. The reason I ask is because most people do military first and it takes forever for them to rank up. For example 7 years to make Sargent in a common case. But then again how would the military be like if everybody went to college first. Would we have a military full of officers? Why not go to college first?