First, there are a lot of nurses who are not working. I don't want to be sexist (which means that I will be) but nurses tend to be female, tend to be married, and tend to have families. I know a number of nurses in this category who have decided not to work, or more specifically to not work inpatient units. The same is true for physicians, but to a far lesser extent. That of course does not describe every situation, but it explains why the inpatient nursing pool has decreased, while the number of working physicians essentially hasn't.
Second, and I will get killed by the hospitalist for this, the nursing workload per patient has increased more than the physician workload. Based on an N=1 - and I am willing to admit I and my source are wrong - the patients that had a ton of physician work (placement) but required minimal nursing care aren't nearly as prevalent.