I think the root of the phenomenon is that we can only communicate to others what we know, not what we understand.
Also, it is not unreasonable to think that one's mental images are not going to be of any help to others (In fact, they may well make things more complicated, or confusing for others: I have been told mental images by others---sometimes indirectly, by the choice of the word introduced in a definition---and been thereby misled; here «misled» means «led in a direction different to the one I personally would follow in order to form my own mental image of the concept».) For example, for me resolving the singularities of algebraic varieties makes a clicking (or clacking) sound: this is quite significant for me in a way, but when talking to others I doubt I'd make any mention of this, for seriously doubt it would help :)
