6 Answers

When many of us were children, respect for our parents wasn't an option.  They were accorded respect simply because they said so (in a variety of ways). 
Today's parents have a tough row to hoe.  It isn't bad enough that schools, state, and federal government have taken away many of their responsibilities and obligations (feeding them, medical concerns for example); In California, laws have recently been signed by Governor Moonbeam that prohibit <18 from tanning establishments REGARDLESS of parental choice. A child who is pregnant may terminate her pregnancy WITHOUT parental notification or consent.  There is a "vaccine" to prevent STDs, which minors also may get WITHOUT parental notification or consent.  Why respect your parents when you don't have to get their permission for important stuff and their permission for other stuff doesn't even matter?
Then there are the parents who deserve no respect.  Basically, parents who have no expectations of their children, are constantly in conflict with authority, and provide little or nothing to their children's well-being (mental, physical, emotional, social, spiritual) don't deserve respect. 
Even a parent needs to earn respect.