Surely you can see that's a recipe for disaster?
People are very bad at judging risks - and everybody things they're cleverer/more careful/better than the average.
That's exactly why no government that I'm aware of has ever enacted what you dscribed.
Ok - so you make it that driving whilst drunk is OK - and you will only be charged if you kill/maim somebody. You will soon get a whole bunch of young 20 year old males, who think they are invincible (or that they can hold their liquor like Captain America) who end up destroying families in completely preventable MVAs.
Everybody thinks they're a "better driver than the average" - or that "their reflexes are really good". Unfortunately, the average is exactly that - the average.
Sure, according to you - you can now charge them after having killed a mum and her kid walking across the road - but what about the fact that you've essentially encouraged this behaviour, and raised the probability of such things happening?
Similar logic applies to any risk taking behaviour - yes, there are certainly grey areas - but there's a reason handguns are a restricted item and not anybody can buy them at any corner store. Having a gun in and of itself doesn't "harm somebody", as you say - but having more handguns distributed among the population, without any attempt to reduce the risk (e.g. background checks, safety checks, licensing checks) will increase the risk that something bad will happen.
Likewise drugs are a controlled item.
And with public health directives like this - the aim is to force people to take sensible precautions to protect those vulnerable among the population. I hate to say this, but people are sometimes selfish and only think of themselves (we all do this). So sometime you need somebody to look out for society as a whole.