I'm sure there are ICU nurses who know practically as much as the doctors they are working with. And that is awesome. But for everyone one of those, there are 9 other nurses who are just punching a time card, and this is why nursing is only a quasi-profession with some admittedly very professional sub-specialties.
It's related to experience and education too. Many nurses get 2 year degrees, and these people are not specializing in anything except general ward nursing. You have to understand the difference in this type of education and a BS in nursing. It's just a job to them. The 2 year programs do not teach the conceptual underpinnings of medicine, and they shouldn't need to, the job doesn't require it. It is much more procedural, "on-the-job" type training, and heavy on following guidelines, i.e. "Give drug X for symtpoms A, B and C. Dose for adults is this. Dose for kids is this. Rinse, repeat."
If you ask your average newly graduated nurse with 2 year degree what 5-HT3 vagal inhibition accomplishes, they won't know, but they probably inject people with Zofran every day. If you ask them what ACE inhibition is, they won't know, but probably give lisinopril all the time. More specialized nurses are going to know a lot more, obviously.
As a doctor, you are part of a gigantic array of resources that you use to practice, and no doctor knows everything. However, I would say that doctors consistently are more familiar with the science behind the functioning of the body, because they have to be. While some nurses may be quite knowledgeable, it's just not the same job. It requires less intellect, and it is more menial work. Notice I did not say nurses "are dumber" than doctors. I just said, the job does not REQUIRE the level of intellect needed by a physician. A nurse could be walking around with entire medical volumes memorized....it doesn't change the job description - the work is still more menial with less thinking required. And I like thinking. Of course, it also happens to pay very well, the schooling costs far less than medicine, and you don't have to give up a substantial portion of your life to your career. It's a personal choice, and I'm not going to try and be "politically correct" about it. I just don't like the work and would be unhappy taking orders and not being the one figuring things out for myself, that's why "medicine and not nursing" for me. Don't most of us share this feeling?