Marijuana has a long half life. One joint may not impair immediately afterward as much as one glass of wine. But pot smokers don't just smoke one joint. They smoke several a week, or more commonly, several each day.
But three joints a day will accumulate and lead to measurable cognitive changes. In other words, three joints a day would be like having a constant blood alcohol level of 0.1mg-- which a normal person could establish by drinking one or two shots of whiskey every hour all day and night. We have a name for such people: Alcoholics.
In other words, like many of the early "pot is harmless" arguments, such comparisons are meaningless.
Don't drinkers also take more than one beer a week? Sometimes more than one drink a day?
Of course, driving while impaired for any reason is a problem. Driving with a migrane is a problem too.
Says who? And, so what?
There is absolutely no statistical evidence to back up your assertion, and, based on your own comments about the persistent effects of marijuana, anyone who did so would be high pretty much all the time. While this may be "common" among small groups on some college campuses, it is hardly "common" among the general population. You should be a little more careful with your assertions.