It is axiomatic. Maybe I could prove it directly, but as an axiom, it is more productive than "some people are smart, some people are stupid".
For example, when you look at children, or look at history, or look at other cultures, you see patterns that make more sense when you start with this axiom. It's not perfect, but if you have a concept of "smart", it is better to assume all people are smart, to take it off the table entirely, than to assume some are smart and some are stupid. The latter will shortcut your reasoning: "Oh well they must just be stupid."
For example, when you look at children, or look at history, or look at other cultures, you see patterns that make more sense when you start with this axiom. It's not perfect, but if you have a concept of "smart", it is better to assume all people are smart, to take it off the table entirely, than to assume some are smart and some are stupid. The latter will shortcut your reasoning: "Oh well they must just be stupid."