"88% of graduates say they were happy with the investment they made."
Of course you will rationalize your decision and you will say you are happy after sinking so much money in! Can you imagine answering something else? The minute you were accepted you were 'happy with your decision' and you crafted and told yourself a nice narrative.
I'm sure there is tangible ROI for a lot of MBA graduates, but the question whether they are 'happy' is simply biased.