Because you subscribe to communities that are full of old memes and/or because you browse 'all' instead of your subscribed feed.

As to why the wider Lemmyverse has too much of that sort of stuff, it's because a lot of people are trying to recreate Reddit instead of building something new. Or because they're trying to kickstart the something new by posting plenty of content and, inevitably, much of it is old.

Post stuff you want to see. Engage with stuff you want to see.