Peds, internal medicine, and family medicine are 3 year residencies after medical school. Some would also include obgyn and psychiatry (both 4 years) in this list.
I obviously don't know about everywhere and every different specialty, but at the two large academic teaching hospitals where I've been, NPs in my department had to staff 100% of their patients with their attending physician, and sometimes they would see the more difficult patients together. They had similar responsibilities as 2nd year residents.
As you said, NPs often are assigned more straight forward cases, and there's no reason well-trained NPs can't provide excellent quality care for routine things independently. But part of it is a prestige thing. Some patients have expectations when they go to a big name hospital that they will be seen by a world renowned physician. There was some talk at my last place to allow a couple of experienced NPs to manage a small inpatient caseload on their own, but it was shot down, I think mostly because it didn't look good.
As far as the cost goes, yeah, it would be cheaper if NPs all worked on their own without supervision, but not everywhere is always 100% profit driven, and NPs are still much cheaper than hiring additional physicians. A lot of doctors at academic centers are not seeing patients all day anyway. Many may only have a 50% clinical appointment. Many will have time allotted for supervising residents/NPs, teaching classes, doing research, and other admin tasks.
I was never try to disparage anyone, so sorry if it came across that way. Just was pointing out the most competent mid levels I've worked with have been in these type of environments. Being surrounded by bright minds, getting quality supervision on a daily basis, sitting in classes/journal clubs with residents, going to grand rounds, etc seems like an easier path to greatness than just doing 500 clinical hours and going at it on your own.