Miscellaneous: What Ever Happened to the Expression "There are?"

I shouldn't let grammar errors bother me, but my mother was a school teacher, one of my grandmothers was a school teacher--in fact, a teacher of English and English literature--and one of my grandfathers was a teacher (of art history). Let's just say it's in my blood.

We all make mistakes. God knows, I make mistakes. I try to correct them when I can, but some mistakes-- for one reason or another--can't be corrected.

But back to grammar errors. One common mistake that baffles me is the use of "there is" or "there's" to mean "there are" or its informal spoken contraction "there're." If people can count, they ought to be able to get this one right. Many who should know better say things like "There's three on the table" or "There's so many reasons to go." I hear this with increasing frequency. It's almost beginning to sound normal. When did saying things like "There're three on the table" go AWOL? And why?