The problem with making laws against people using drugs and driving is that there is no field test for, say, marijuana like there is for alcohol. You can pull someone over and determine immediately if they are drunk above the legal limit by using a breathalyzer. There is no such test for pot (or other drugs). How would laws against driving while high be enforced?
The effects of being high on drugs are fairly simple to spot, the first port of call would be the pupils. Dilated pupils are usually present when a person is high. Reaction times are typically slower. Abnormal behavioural tics may be present. If a person displays these traits, a simple blood test/hair and fibre test, in the case of inhalants, can show the presence of drugs.
However, it is possible to drink responsibly. It is not really possible to smoke crack responsibly.
Arguably, if a person is smoking crack of their own accord and doesn't drive, operate machinery, supervise a dependant or otherwise engage in activity that may harm another, then that's reasonably responsible. The harm they may be doing to their own body is entirely their own problem.
Legalise 'em, tax 'em, provide safe and non-judgemental spaces for those who become dangerously addicted to quit, a la alcoholics anonymous/quit smoking groups. If drug use can be monitored, we can do more about it if it becomes a problem for the user. I can't stand drugs, nicotine or alcohol, but they're all of the same ilk, and it seems odd to make two of those legal but not the other.