I think it is because of a few interrelated things: 1. Ill-posedness of the problems: what is sleep?, why do animals sleep?, etc. vs. the well-posed physics pr

I think it is because of a few interrelated things: 1. Ill-posedness of the problems: what is sleep?, why do animals sleep?, etc. vs. the well-posed physics problems such as, what happened at the beginning of the universe? 2. Data sufficiency: there is too much variability across people and animals to have enough representative data marginalized on each variable for many biological systems in general, and sleep in particular 3. Complexity of the system: cosmological systems and even subatomic particles are less complex models than biological systems On incentives for researchers, there is also the maxim that there are many unsolved and interesting problems to work on, but scientific judgement is in knowing which are solvable. Of course, the occasional maverick goes after the hardest, most ill-posed, most unsolvable problem. But the existence of such mavericks is not guaranteed for every problem, sleep biology included.