A lot of non-brevity is because the way we communicate naturally (in person) uses numerous channels at once, eg body language, intonation, and the actual words. Studies show that the actual words convey only about 7% of the information sent, AND that words are often disbelieved because they are so often used as lies. Think about it: when your girlfriend is upset, and you ask her what’s wrong, and she says, “nothing,” you know this is untrue; she’s probably mad at YOU for something you didn’t even know you did.
You are probably also aware of something called a “flamewar”, especially with Facebook. this is when a text-only conversation erupts into an argument, usually a very angry argument, and is almost always caused because one person misunderstood what the other was saying and assumed insult. A huge argument, filled with insults and hurt feelings results, and often the people involved get banned, even if they were right.
You probably also have seen in the news where someone said something perfectly innocent, like describing a bad situation that can only be made worse as being a quagmire. Frex, Vietnam was a quagmire. However, recently someone wanting to use a word other than quagmire used a synonym for it: tar-baby. And guess what? Someone who wanted to take offense did so, blew it all out of proportion, and now we have things like “political correctness” in an effort to protect ourselves from over-sensitive idiots who are in reality passive-aggressive bullies (but that’s for a different day).
The point is, context matters, and in text, you can’t convey context with body language or intonation. You often can’t even convey it with emphasis (italics or all-caps) for fear someone will accuse you of shouting and thereby miss your point.
The second big reason (and this will be much more brief) is because some of us crave clarity. Omitting description leaves room for multiple interpretations. When we’re trying to communicate things, multiple interpretations are usually NOT desired (unless we’re con artists, and then we do want our victim to misunderstand what we really want).
If I say “the asteroid is as big as a house”, that’s not very helpful. One listener may think of a small house, another may think of a large one. Who knows what the speaker actually intended?
So yes, many a time, we do inflate our explanations in order to make ourselves more important, but also many times that extra explanation IS important to conveying the meaning. (And in this case, it DOES prove how much more important I am. )