Venom is huge, his character takes time to be born. They need to advertise him. If they have Brock and the symbiote in 3 (not Venom), this would make an awesome cliff-hanger for people to go see 4 (and make 4 one of the first movies to make the fourth film better than the rest).
Jensen Ackles for MCU Venom!
My take on a new Spider-Man trilogy
1st movie: Kingpin and Vulture with Eddie Brock in a minor role
2nd movie: Kraven's Last Hunt with a Sin-Eater subplot, Spidey having the symbiote from Infinity War
3rd movie: Venom