One reason why is that dental hygiene is four years, two years pre-reqs, two years of the program. There are also additional pre-reqs required for dental school, so you would have to do hygiene on top of pre-reqs. Then you would have to study for hygiene boards and DAT at the same time. Then, you finally graduate as a hygienist, but wait, you can't work because you just got accepted into dental school. So now, all your hygiene time is really all for not (sure you have some dental experience, but it is hygiene, which is good, but different than what the dentist does. In my opinion, you have two 'offices' in your practice. You cover restorative, prostho, oral surgery, endo and ortho. Your hygienist takes care of perio. She is the specialist in the office in that area.)
I don't think you would have time to make money as a hygienist in dental school, so basically it is a lot of work for being in the exact same position as your fellow dental students that have a lot of time shadowing dentists. Maybe you will have an increased appreciation for hygiene though. And get A's in perio....