Why is it so many otherwise bright-enough individuals persist in the dogma that any single integer can meaningfully aggregate all the vastly diverse skills of a

Why is it so many otherwise bright-enough individuals persist in the dogma that any single integer can meaningfully aggregate all the vastly diverse skills of a human brain -- when they would hesitate to do the same with an object as simple as a bottle of red wine? This remains to me a deep mystery of human psychology, one I've pondered off and on for some years now.