No one has said it, so I will state the obvious.
BECAUSE MILLENNIALS ARE NARCISSISTS!!!
Millennials didn't even notice you when you sneezed - they're in their own self-centered world of thought. If you paid $15/hr or handed them trophies for saying "bless you" they would oblige, maybe. And the point about the understood subject "God" - yeah, Millennials are far too arrogant to even consider that there might be a god calling the shots. Their health is in their own hands (and Obama's), thankyouverymuch. Millennials' social skills are so underdeveloped, they can't even socialize without alcohol.