Ian ( snipped-for-privacy@btinternet.com) gurgled happily, sounding much like they were saying :
...?
Because the very effect of alcohol is to impair your judgement. It makes the Neils of this world think their driving is fine, even when it isn't. By definition, you are the last person who can judge your suitability to drive after a drink.
If the conditions change whilst you are driving at a speed above the limit, you can slow down almost instantly. You can't sober up almost instantly, even if you recognise the need to.
Yes, I know that's not quite your point - which is that there's an arbitrary fixed number applicable to all regardless of tolerance, body type etc etc. Other than the utterly hokey 'merkin-style "Can you walk in a straight line without falling over" test, though, I'm not quite sure what the solution is.