They just don't get it. They are fixated on "one cause-one disease-one cure" thinking and don't entertain the possibility of something far more complex like "exposure to A sets initial conditions, nothing happens unless B is encountered in which case system is primed, if two or more of C through M impact the system simultaneously then a cascade is triggered with the ultimate disease being determined by the particular combination that initiate the process"

Something along those lines is needed to account for the fact that PD is sporadic yet common but very individualized. Tweak a variable or two and you've got a different condition but it is still messing with your brain and it is still degenerative and it still strikes after mid-life etc.