I'm not saying humanity is impossible to describe (on the contrary!). your physics example is too different to what happens in social sciences, it's not a good analogy. you can largely control the conditions under which you throw the balls, while if you're, for instance, studying the recent socio political events in the USA, things are given and then you need to rely on similar past event to understand current ones. even in fields like psychology it's too difficult to even measure initial conditions and outputs. that's why you rely on qualitative analysis.