A few words are related to sound: in English, "meow" sort of sounds like the sound cats make (although my cat is much more nuanced, he wants you to know!). "Woof" sort of sounds like some dogs' bark, "cock-a-doodle-do" has the same stress pattern as some roosters, and so forth.
Some languages tend to have more onomatopoeic sounds than English, and signs in sign language (especially "young" sign languages, meaning sign languages that have been codified recently) are probably even more onomatopoeic.
But in general, your point is true: the vast majority of words do not sound like the thing they represent. "Knock" (as in a knock on the door) doesn't sound anything like a knock.
Some languages tend to have more onomatopoeic sounds than English, and signs in sign language (especially "young" sign languages, meaning sign languages that have been codified recently) are probably even more onomatopoeic.
But in general, your point is true: the vast majority of words do not sound like the thing they represent. "Knock" (as in a knock on the door) doesn't sound anything like a knock.