Venom's superhuman strength and symbiote abilities overwhelm Captain America's peak-human combat skills.
Captain America is one of the most skilled fighters in Marvel, but Venom represents a massive physical mismatch. The symbiote grants strength exceeding Spider-Man's, shapeshifting tendrils, and a healing factor. Cap's vibranium shield is his greatest asset, but Venom can attack from multiple angles simultaneously with tendrils, making the shield's directional protection less effective.
Cap's combat skill and tactical mind are superior, and the vibranium shield can certainly hurt Venom with well-placed throws. But the symbiote's durability and regeneration mean those hits do not accumulate effectively. Venom can absorb punishment that would stop most opponents and keep coming.
Captain America would need to exploit Venom's fire and sonic weaknesses, and his standard loadout does not include those tools. Without specific anti-symbiote weapons, Cap is fighting a physically superior opponent who can attack from impossible angles and regenerates from most damage.
Captain America's vibranium shield, combat skill, and tactical genius have beaten physically superior opponents before. His leadership instincts and ability to find weaknesses mid-fight give him a chance against any opponent.
Venom's strength dwarfs Captain America's enhanced physiology, and symbiote tendrils can bypass the shield's protection. The healing factor means Cap's attacks do not accumulate, and Venom can engulf and overpower Cap physically.
Venom wins through overwhelming physical superiority. Captain America's skill and shield keep him alive longer than most, but without anti-symbiote weapons, he lacks the tools to win against a superhuman opponent of this caliber.
Captain America also fights
Venom also fights