Many people are genuinely not "smart" enough. Saying they are, is likely to cause suffering and waste resources, on both personal and societal levels. It's also insulting towards the intellectually disabled.

"I'm not smart enough" is not always a cop-out, or "a (lame) excuse". It can be a sign of maturity, and a cause of significant respite. Knowing your limits allows those who are intellectually limited to pursue tasks suitable to them, and still be satisfied with life (even though they haven't created Facebook or Pinterest).

Even for the more gifted of us, knowing your limits might lead to a happier life..