The context in which a word is used is typically more informative than the meaning in the dictionary. For example, "set" has an unreasonably large number of definitions [0] but I can't remember the last time its usage in a sentence was confusing.

It's also why "cromulent" from The Simpsons had a clear meaning during the episode that coined the word even though it did not exist prior to the episode airing.

[0]: https://www.merriam-webster.com/dictionary/set

So... you're nonplussed about why people would pick up new words from casual usage rather than from books?

That's how we learn 95% of words' meanings: by osmosis from hearing them used by others whom we presume know their proper meanings. I doubt you've done 100,000 dictionary look-ups, or any number remotely in the ballpark of the number of English words you know.

How did you learn your first language? Most people learn to speak before they learn to read and use a dictionary.