Why do we say more than is necessary? Why do we write more than is needed to explain or convey the message?
We may not know the topic well enough. We may understand it enough to apply ourselves, but probably don't know it well enough in a "taught" format.
We may feel we need to add a lot more information because the other person may not have all the context or background information. We may feel like providing more information because we personally find it interesting.
And for all these things "we may" be or "it may" be, the only thing that's clear is we don't know ourselves or other people perfectly to know exactly the right information, said or written in the right way, so that our message is anything but concise.