Hope/expectation/desire to fit in: I just paid a lot of money for these fancy new cranks, and, oh, yeah, they're so much stiffer! Or: Everyone says these cranks are stiffer than those other ones, and, oh, yeah, I can feel it too!

Or, less cynically: The crank itself is rarely the only thing to change. There's also usually a clean bb installation, new chainring, etc., plus just a general tune-up of the whole drivetrain. So all of that can cause a real and noticeable mechanical difference that people might misattribute to the most salient change that was made--i.e., the installation of the expensive new crank that drove all of that other maintenance--but finally that difference isn't down to the stiffness of the crank.