If so many people weren’t leaving the field entirely due this issue (the chief complaint ALWAYS being under-staffing / low nurse-to-patient ratios, THEN pay), there would be plenty of nurses to go around

I think both can be true.

From expenses point of view, Isn't under-staffing almost the same thing as low pay? What's preventing hospital administrators from hiring more nurses? If it's just money, then I don't think the complaint of under-staffing all that different from the complaint of low pay; I suspect it's even affected by sort of preference (some nurses would prefer working more for better pay, others would prefer sharing the workload.)

Of course from administration / governance point of view it boils down to money, what I'm saying is that I find it unlikely is that it's "just hire more nurses". It's also doctors, other staff, etc. It's more likely the whole system.