People forget or don't keep up to date on rules. Some kind of retaking the theoretical parts would at least be nice. For instance the UK has a new highway code (I've heard), and most countries it's quite different to drive now than lets say 20 years ago. For instance the amount of cyclists and the rules around that I feel drivers in Norway have a bad grasp on.

And heck, when I got my license I lived in a remote town, 3 hours away from the nearest street light. Lots of things I got away with not learning, and whatever I memorized for the theoretical test 20 years ago is mostly forgotten. I think many people think they "know" the rules, but really don't

You also have the problem that the pass rate for tests has changed massively in the years since most 50 year olds took their tests in the UK. Anecdotally, it is not at all uncommon for young drivers to fail the test 3+ times (at least amoung the youngsters I know). 30 years ago it was rare to have to take the test more than 2x before passing.

That and the scandal of test slots being bought up months in advance by what are essentially touts, as well as the lack of testers you noted, make it unworkable.

There could be, it's just that we don't value testing highly enough and we're happy to see people killed and seriously injured by these things.