That's an interesting point; what is a given for good driving in terms of ability or psychological fitness?
Can we measure it?
If so, why don't we do this, given how dangerous it is to drive a 2 ton lump of metal about at high speed?
Ability, the standard legally tested caselaw is do they fall below/well below of the standard expected of a competent driver. "standard" being level to pass driving test. psychological fitness that's harder psychological profiling has it's fair share of holes so won't really work.
Advanced/high speed driving the goal posts changed the other year legal standard moved from normal driving test to an advanced one.
Making it harder to prove fault.
Currently any of theses test only apply once something happens and if chargers are made. Driving test is like any other exam pass on the day is no guarantee of future performance. Some form of continuous assessment and portfolio maybe as part of a bigger solution. Treating it more as a vocational skill than simple academic pass/fail exam.