Probably the Most Obvious Answer. Ever.

Humor me and let's play the question and answer game for this post.

Q: OK, so there are hoards of people around the world who aspire to become nurses... for some reason. Fine. It's good to have goals. And apparently, there's a nursing shortage. So with all these bright-eyed and bushy-tailed "save the world" types going into and graduating from nursing school, why, I ask you WHY, is there a nursing shortage?

A: Yes, I will be the first to say that there are a hell of a lot of reasons why there is a shortage. Too few faculty and so on and so forth. However, if you get down to the bottom of it all, I'm talking the lowest, blackest point of the abyss, you'll find that the reason is that NURSING SUCKS. And all those who have left the practice are testaments to it.

Let me revisit the tale of the bright-eyed and bushy-tailed nursing student. She's/he's breezing through clinicals and actually enjoying it. She/he did an internship and it went swimmingly as well. Uh-oh, here comes graduation (assuming she/he does make it through) and after a stint in an orientation, she/he is responsible for a hell of a lot.
She/he soon grows disillusioned by the realities of the business of health care. The bureaucratic shit that they don't concentrate on in nursing school for a reason; the burnout from all the emotional, physical, spiritual, and literal shit you have to clean up q shift; ridiculously catty co-workers; families that annoy the hell out of you; the list goes on and on. Remember Maslow? Well, replace the top of the pyramid with "realization of the suckiness of nursing" and that's where you'll find this nurse.

Time and time again, I've heard many a nurse, senior and fesh meat alike, voice their desire to leave the profession. And hey, who can blame them? So they leave for work that is more fulfulling and much more enjoyable. Ergo a drop in the nursing workforce. A drop in numbers faster than can be replaced by the naiive, albeit well meaning (mostly), nursing schoolers. And finally, an answer to the original question.

The fact is, you can't pay most people to do the shit that nurses do. It's a dirty job.