While the "elves spend most of their long lives in leisure" explanation is kinda nice and Tolkien-esque, it doesn't solve everything to do with their lifespan.

Imagine you have an event in your setting that took place 1500 years ago. That's as far back in time as the fall of the Roman empire is from the modern day. In real life that's a long enough time for multiple empires to rise and fall, for language to evolve to the point that speakers can no longer understand the previous tongue, and for people to change their religion and forget they were ever pagan to begin with.

Elves in DnD live 750 years. A 200 year old elf PC could reasonably say "wait what if my grandpa was there? DM do I remember my grandpa ever talking about this?"

This is a result of taking something that should be awe inspiring and making it mundane (letting people play as elves). And it's not the only instance of that in DnD.