I have a hard time squaring that with 15 year fixed rate mortgages being under 3% for several years in the very recent past and 30 year fixed mortgages being at/under 3% for part of that time.

How would an adjustable rate mortgage be the best choice for consumers in that situation?

I got an initial mortgage at 5.625%, refi’d to 3.875%, then again to some ~3% 15-year. I never seriously considered an ARM (and now wish I'd refi'd a third time back to a fresh 30-year fixed just to keep using that cheap money for longer).