Those sites are centralized and governed by a single private company, so they can have a dedicated team of content moderators to check and verify illegal content. Federation and how lemmy caches content on all federated instances means when someone posts illegal stuff on one instance, every other connected instance gets a copy too. So random guy running an instance in his room has to go find the content and purge it themselves even though it wasn’t posted directly on their instance, or they can be held legally liable depending on the country they’re in. It’s also the reason (whether you agree with it or not) why instances like .world preemptively blocked piracy communities.

They use specific tools connected to a database that automatically detect and disallow that material from being posted.

If something like that gets posted to reddit, it is in a single database (even if it is reduntant and/or distributed) controlled by a single entity and can be removed. Cleanup is a breeze.

In Lemmy, since servers cache federated media, it gets saved across multiple servers. Every single admin has to be notified and then manually do the cleanup on their server.