It's easy to make a job look good for a day, but on some jobs if you skip the prep work, lining, filling, priming, undercoat, additional coats, clearcoat, and finishing the paint job is going to look pretty bad after a year. You can't tell how good a painter is unless you know a bit about painting things.
The same is true of software. In order to know if a developer is productive you really need to know quite a lot about the domain they're writing software for.
I manage a team of developers / analysts and it's straightforward (though not always easy) to measure this over several six-week blocks of time for the team. It is not, however, easy to measure it for any single developer or analyst, and there's a lot of variance when you try to look any more granular than the team.
I suspect it's doable for small groups that work together. Or even larger groups. (I have no experience here.)
But for individuals? I agree. No shot.
If I hire painters, I can tell if they’re doing a good job by looking at it. I know what’s taking them so long. They’re using slower drying drywall compound, or they’re having to mask off because they’re not good with a brush.
There are fields of work where it’s easy to gauge productivity, but software development is not one of them.