Content aggregators are not forums. Just having categories doesn’t really cover it. CAs are designed so that old posts fall away quickly, so that people will keep posting new top level content and keep people emgaged in the constant scroll, much like Twitter or Facebook. They are largely unstructured, with different “categories” behaving quasi-independently from one another.

Forums are structured spaces where the same people post stuff to the same categories, that are mostly offshoots of the forum’s core theme.

People interact with and behave rather differently in these different contexts.