That's only because it's consistently misused by those that lack intelligence. The original and true meaning of the word is inability to speak until it was 'dumbed down' because of misuse. Kind of like how 'cringe' has suddenly become an adjective because someone thought it sounded cool or something.
In my search, dumb as you define it, shows up as the 3rd entry and also as 'Informal North American'- so basically it's the wrong definition but it's accepted because people are too stupid to know better.
adjective:
dumb; comparative adjective:
dumber; superlative adjective:
dumbest
- 1.
temporarily unable or unwilling to speak.
"they stood dumb while the attacker poured out a stream of abuse
2.
OFFENSIVE•DATED
(of a person) unable to speak, most typically because of congenital deafness.
"he was born deaf, dumb, and blind"
3.
INFORMAL•NORTH AMERICAN
stupid.
"a dumb question"