Practical speaking versus aesthetic speaking.
Practical speaking is getting the message across using simple sentences and common words. Aesthetic speaking is getting the message across using complex sentence and unusual words but without being verbose or ambiguous. Neither form of speaking is incorrect or more preferable than the other.
Practical speaking – it was a beautiful day, it was a nice day. Aesthetic speaking – it was an exquisite day, it was a picturesque day, it was a gorgeous day, it was a magnificent day, it was a wonderful day…
Though observation of conversations, participation in conversations and reading newspapers, magazines and posts on internet forums, as well as discussions about word usage, I've noticed that people understand a large range of words but seldom use the many words available to them. I've come up with some possible reasons.
This is based on my observations of the use of English in various communicative situations.
A person doesn't know the definition of the word so is unsure of when to use it or how to use it. This could be because they have not come across it before or they have read the word but are unsure about how it is pronounced thus going with a more common word that, while having the same or similar meaning, will lessen the chance of embarrassment or discomfort if mispronounced. A word may not be used because it has negative connotations or it is found to be cumbersome in everyday conversation; the word may not be used because it might associate someone with a group or social class they do not want to be associated with. So even if the word is understood and can and might have even been used, the association to something would prevent that word from being used. In contrast, some words may not be used as to maintain associations with a certain group or social class rather than to disassociate.
English might not be the user's first language and so they are hesitant to use words that they might use incorrectly or use words that they do not know the actual meaning of. Meaning might have been gained through previous conversations where the content and the context explained the word but the true meaning is not known. To prevent incorrect use of the word, it is not used at all.
People may not use the word because they feel they should not be able to use the word. Either due to education level or social standing or some other idea that prevents them from being comfortable with the idea that they are 'allowed' to use certain words. So even though they understand and are able to use a range of words, they fall back to the words they feel they should only be able to use rather than those they are capable of using.
There is also the possibility that people do not use words that have special meaning to them as they do not think that the listener will fully understand the emotion they associate with the word. They, therefore, go with a more common word to impart mutual understanding instead of having the listener focus on the word rather than the emotion they are trying to impart.
Subconsciously some people may use words they feel their listener will understand. More common words aid the interpretation of what is being said and are used avoid confusion or misunderstanding. In this case the word choice is to facilitate correct understanding of the message.
And finally, there is laziness. It is easy to use the words that come easily when speaking. Instead of using the words that they know but require a little effort to use.
So while there may be a number of reasons for people to not use the words they know in a given situation, it does not mean they do not know the words or how to use the words.