Unity does not make that much money from assets. They make the majority of their revenue through licensing their engine to "whales"... the small percentage of games that make huge revenue.

They also make money through ad services... a market they seriously missed out on (look at AppLovin stock vs Unity).

Asset sales are barely a blip.

Unreal makes the vast majority of it's revenue through microtransactions on its one major whale game Fortnite