I am not defending those that return 5 phones. But *this* could explain it....
Your first phone goes bad. Now you're more "attune" to potential problems
With each new phone you get - you become more and more picky/notice other things - or are more sensitive the SECOND something seems out of place or has a glitch.
It's a snowball effect I think. That's why many of these people are really just looking for ANY thing remotely wrong with the phone.