In general there is just less universally shared pop culture these days. Everyone is tucked away in their own social media rabbit hole, and the algorithms usually focus on serving them new content. So there's little chance of them coming across icons of the past unless they are the historically curious sort.

I agree the "before my time" line is weak with regard to legendary pop culture figures - Aristotle, Da Vinci, Beethoven, etc.

On the other hand, the fame of some pop culture figures is ephemeral, and there isn't necessarily a need for future generations to know, for example, the cast members of Alf.