Are you sure why they wouldn't? Which is to say that if they are functionally indistinguishable then what is the difference other then the assumptions required? Isn’t a reality that is materialistic as well as functionally materialistic more, well, parsimonious than an idealistic reality that just seems to be functionally materialistic? Without solipsism (as I think the OP asserts) idealism gets even less “parsimonious” as some overseeing, greater or combined consciousness has to be introduced, and evidently just assumed. However, the problem still remains, why should there be any apparent material restrictions at all, if even idealism wasn’t at least functionally materialistic?