Why are the terms "Mr" and Mrs" ____ usually referred to the matriarch and patriarch of a family, even when they have sons who are married?

Yes, that's why. Or maybe its just that in their day, that is how they would have been  addressed and its just continued over the years? 

I have no idea what you're asking about.  That's nothing new.  Maybe it's a sign of respect.  You call your grandparents Mr. and Mrs. Wood?

This is your 484th answer. This happens to be the pattern in your family, but don't generalize to everyone else. It just isn't so for everyone.

It used to be customary to call an adult older than you Mr. --- or Mrs.--- or Miss --- until you were asked by that person you to address him or her by their first name.

Maybe your parents have already made it clear that was okay.

Where I live, married people are referred to as Mr. and Mrs regardless of their age or position in their families. It is the norm. 

Mr and Mrs are any married couple of any age, with or without adult children maybe your family is just weird. Most kids call their grandparents grandma and grandpa not Mr and Mrs