They don't? Why did the Carnage sample mixed with Peter's blood create a Parker-like being. And why is Gwen, a victim of Carnage, back now? I thought that's what they did.
Because Carnage isn't a symboite.
A symbiote is something that lives as part of another living being. It's like a parasite except it "gives something back" to the host...the symbiote and host live in harmony together, each giving and taking equally.
The original 616 Venom suit was a symbiote. So was Carnage. Neither Ultimate Venom nor Ultimate Carnage fit that description.