I don't think people have the thought process that the flush draw which will most likely scare them (a reason they'd bet turn) gets in on the river so I best overbet the pot. There just doesn't seem to be any reason to.

I do think you're stacking him if he has any flush btw, I just doubt he has that many flush combos in his range and for all the ones he does it makes 44 & 22 more likely imo.