What makes you think they didn't get more stupid? The illegitimacy rate shot up. The savings rate fell a lot. Does whatever makes people fat also turn them into irresponsible parents and poor retirement planners?

My hypothesis is that we've seen a weakening of social norms and pressure, as reflected in the illegitimacy rate and the deterioration of standards for clothing in public. You faced more opprobrium for being fat, bearing a bastard child, or dressing in cheap garbage in the past. Now you can comfortably be fat in public, and wear flip-flops too.