I don’t think that more people find and keep the whole sand dollars.  That isn’t why they’ve disappeared from the beaches, but rather that changes in the weather or the health of the environment has caused the shift.  And yet, this shift is probably the result of the increased flux of humans and their habits.  

Maybe the marine creatures that live in the sand dollars have moved further off coast, causing the shells to be more brutalized and broken by the time they reach shore.  Or, there are less of them to begin with, or more of whatever feeds on them.  Whatever it is, it is rare to find one even almost intact.