Not only is it part of science, it is the logical conclusion to fundamental scientific theory. All other scientific theories feed into a theory of everything - the idea that the laws (or a single law) of the universe determine the choices that explain all phenomena in the universe including the choices that humans make (some might say already arguably the logical conclusion to GR). While you might be right to say that it's yet to be 'proven', - and while there are issues with it as @chippyboy already mentioned, that's not to say there won't ever be a theory that unifies QM and GR and that's not to say that many scientific observations or laws already theorised by geneticists, physicists and even behavioural economists don't do a far better job of explaining human behaviour and are much more scientifically valid than the idea of free-will, God or 'hard-work' or anything else it can be called.
Logical conclusions and inference are not science nor scientific.