Can you move while tossing the ball in the air repeatedly (dribbling against the sky)?
No.
Wait why? It's just like dribbling except upside down.
It's just the rule.
It's just the rule, man. The rules are what define the game. Why can't I start the bishop where the queen is? It's just the rule. Why can't I have an under inflated ball? Just the rule. Why can't I use a cricket bat in soccer? Just the rule.
I'm not aware of any sport that restricts your diet, or the amount of exercise or training you can perform. Yet some chemical substances are arbitrarily banned. It is anti-science!
Completely agree. The purpose of sport (in our modern day) is well-being. We should not force anyone to (within reason) be expected to hurt themselves.
However, it strikes me that we are failing to use science in a way that could contribute to sport or human advancement in general. The vast majority of modern doping has no negative health outcomes. The steroid era of the 80s are gone. Many of the most pervasive forms of doping, such as blood doping, EPO, etc. naturally occur in your body.
It has gotten so bad that the NCAA does not permit excessive coffee drinking, as it is considered a form of doping. Furthermore, if you permitted these forms of doping, then the marginal advantage of other more dangerous drugs would be reduced.
Perhaps a better question is: why are there rules against doping in the first place? As the xkcd notes, 'some performance enhancements are ok, some are bad', and the line is rather arbitrary at this point.