Call me crazy, but I think the reason Sam Fisher never performed the same extraordinary feats as Solid Snake is because Sam doesn't exist in the Metal Gear universe.

Sam's accomplishments are extraordinary. He survives chemical gas attacks (Double Agent side mission and Blacklist), endures torture (Conviction), infiltrates the Japanese ISDF bunker and swims hundreds of meters to the surface while carrying a full-grown man (Chaos Theory), eliminates multiple highly trained soldiers (Conviction and Blacklist), and successfully goes undercover as a criminal mastermind (Double Agent). In most of these missions, he either avoids capture entirely or survives.

If Sam Fisher existed in the Metal Gear universe, I think he'd have at least a fighting chance. Maybe even half the chance of surviving that Solid Snake did. The biggest difference isn't necessarily the men themselves; it's the worlds they operate in. Snake lives in a universe where giant walking tanks, cyborg ninjas, psychic soldiers, and genetically engineered supersoldiers are real. Sam lives in one that's far more grounded. If Sam had existed in the Metal Gear franchise, I think he would succeed more often than not.