Even if Carnage’s origin wasn’t depicted that faithfully in Venom, it would have been incredibly difficult setting the stage for him at the same time we’re watching how Eddie Brock becomes Venom.
In the comics, the Venom symbiote gave birth to the Carnage symbiote right before breaking Eddie Brock out of prison, and the latter gooey alien soon latched itself onto Eddie’s cellmate, Cletus Kasady.
It’s more important to give Carnage his proper due, even if it means putting him in the spotlight later.
Riot may not be as compelling of a villain, but at least it doesn’t take nearly as long to establish him.