No profession uses only one skill. I’ve had a few, the longest-running as a developer, and there is nothing uniquely varied about working in software. One thing that is pretty unique to software is how superior they feel to most other professionals, when in reality, it’s a pretty mediocre discipline compared to electrical or mechanical engineering, medicine, chemistry…

Cobblers design, make and repair shoes of various kinds, boots for various purposes, slippers and moccasins with leather, cloth, rubber, and many kinds of threads using punches, knives, various machines, glues…

How much does a developer need to learn about their core competency when switching employers? Not even close to as much as a machinist. It’s not a useful comparison.