In every profession that is relatively high stakes, there is a pretty concrete chain of validation that the person can do the thing they claim to be able to do

In every profession that is relatively high stakes, there is a pretty concrete chain of validation that the person can do the thing they claim to be able to do and that chain of validation does indeed rest on demonstrated performance at that capability. Yes, a surgeon doesn't need to perform traial surgeries every time they change jobs. But, also, they only work inside certified, heavily-regulated institutations that are able to vouch for their previous performance. No such institutions exist for software engineering.