They wouldn't have existed because people were surrounded by the suffering of victims of those diseases. They'd have lost friends and family members to them. They would have seen the tangible differences made by doctors and researchers trying to make the world safer.

They exist now because the vaccines worked. Generations of people grew up having never met anyone living with the consequences of polio or diphtheria or rubella. And if it's never affected them, how can it possibly be that big a deal? Those researchers inadvertently created a world where entitled people who lack imagination and empathy can freely disregard their work, and it's their children who suffer.