Because he had a ton of backup and weapons usually and typically is going up against people that have no chance against him because they aren't super human.
Steve was winning their initial fight in WS until he found out it was him, and took him down in their final fight long enough to hack the carrier and probably could have finished it if he wanted to but instead refused to fight him and let Bucky beat him up. And in Civil War he straight up stops Bucky from escaping and knocks him out and captures him. Likewise Panther is always winning in their various fights there (even without his suit) And of course Spiderman easily overpowers him and Falcon has to deal with him.
Bucky was only good when beating up unpowered people like Widow and assassinating random people with his heavy firepower and metal arm.