In the first chapter of JLA’s “Divided We Fall Arc” both Clark and Bruce reveal their civilian identities to the rest of the League. This is post “Tower of Babel” where nobody but Clark still trusts Batman, and in order to start building trust again, Clark urges Bruce to unmask himself to the rest of the team because Bruce obviously knows who everyone else is. Bruce agrees on one condition, Clark has to “unmask” himself as well.
When the big reveal goes down, Kyle Rayner says it best re: Clark being Superman: “He doesn’t…wear a mask. I never even…thought he had a…day job…”
That’s right, the canon reason why nobody makes the connection between Superman and Clark Kent is because nobody thinks that Superman HAS a civilian identity.
Also, with a really good actor, Clark Kenting is entirely possible, as Christopher Reeve demonstrates in the 1978 Superman film.