It is tempting to speculate on what might cause the credulousness of the broader public re: fMRI, but I think there is enough / too much going on here for me to really be able to say anything with much confidence. Scientism especially is hard to define.

I think I broadly agree with you though that credulousness to (statistically and methodologically weak) scientific / technological claims mostly comes down to vibes and desires / needs, and not statistical significance, logical rigor, evidence, or etc.

Where needs / desires are high, vibes will (often) win over rationality, and vice-versa. It is easier for people to be objective about science that doesn't really clearly matter in any obvious direction, or at all. fMRI is "the mind", and thus consciousness, and so unfortunately reduces rational evaluation in much the same way speculation about AI and "consciousness" and etc does. *Shrug*