Thor respects Cap enormously, but the power gap between a super-soldier and a god is enormous.
Cap wielded Mjolnir in Endgame, proving his worthiness โ but that was an exceptional circumstance with Mjolnir's power amplifying him. In a standard fight, the gulf between a peak-human super-soldier and the Norse god of thunder is immense.
Thor can survive the vacuum of space, withstand the force of a neutron star, and shatter mountains with Mjolnir. Cap's vibranium shield is his only equalizer โ it can absorb any kinetic force, including Mjolnir strikes. But Thor has energy attacks (lightning, god-blast) that bypass physical defenses.
Cap's fighting skill and tactical mind are superior, but they can't overcome this kind of power differential. It's the same problem Cap faces against Hulk โ you can be the better fighter and still lose to someone who is simply too powerful to hurt.
Cap's shield absorbs Mjolnir strikes perfectly, and his tactical mind is sharper than Thor's. If Cap can get Mjolnir away from Thor (unlikely but has happened), the fight evens out dramatically. Cap has also proven worthy of Mjolnir โ in the right circumstances, he could turn Thor's own weapon against him.
Thor's strength, durability, and energy projection operate at a tier that makes Cap's super-soldier abilities look quaint. Lightning ignores the shield, and Thor's raw strength can simply overpower Cap's blocking. Thor also has thousands of years more combat experience.
Thor wins through overwhelming divine power. Cap's shield and skill make him survive longer than most, and Thor's deep respect for Steve would make this a reluctant fight. But if both are committed, the power gap is too vast.
Captain America also fights
Thor (Norse) also fights