At this point MC has probably killed something like a hundred thousand enemy combatants, most of whom have shielding of some kind and many of whom are vastly superior to MC in terms of equipment, endurance and tech. Cap physically cannot protect his whole body simultaneously and one of Chief's most common opponents are an alien whose entire body except for one small bit is protected behind an energy shield. Which he dispatches in seconds. If cap is running around at Chief he's going to get bullets to the leg. Unless he turned into superman while I wasn't watching, he's now on the ground, maimed and screaming in pain from having a dozen 7.62x51 NATO rounds embedded in his lower body. If he's not protecting his face, he now has bullet holes where a brain used to be. If he's not protecting an arm, his arm is riddled with bullets.
Even in a fistfight with no armor, Cap would at best be "comparable" with Chief's physical strength. With the power armor suit Chief has a significant advantage. He can move with superhuman speed, endless stamina, and punches extremely hard. You're the comics expert, can Cap punch hard enough to bend materials stronger than steel which are additionally protected by energy shielding capable of resisting explosions?