The idea that Nurse Practitioners and Social Workers "do almost everything that psychiatrists do but (for historical reasons) charge lower rates" doesn't feel like a fair characterization. I have worked with many excellent NPs and Social Workers, and have a tremendous regard for their work, but the level of training is - objectively - much different.
Here's an infographic comparing training, which is more or less accurate, but there's some variation between programs: https://i.redd.it/ixgg0l7v6lv51.jpg