Why don't more professors utilize their students as research help?
I do employ students to participate in my research. In my department these students (Bachelor and Master students) are paid. That is the first limit to the number of students I can employ: I can only employ so many students as my budget allows. 

The second limit is that in the kind of research I do these students cost me more time than they save (unless they are really good). Doing real research is for them a whole new experience, and they have to learn. That costs time, I need to do a lot of explaining, errors happen that need to be found and than fixed, etc. 

That is fine, giving them that experience is why I employ them. They can see if this is something they are interested in pursuing as a long term career. If so, then they start with a bit of experience. If not, they are saved from starting a PhD and than dropping out. Either way is a positive outcome for them. For me, employing students for research is more a teaching task; an enjoyable and rewarding teaching task, but not something that I expect to improve my productivity as a researcher (If it happens than that is a nice bonus). 

This obviously depends on the kind of research one does, and the kind of tasks that needs to be performed to do that research.