It comes from the tautology all that dicourse about conciousness experience (and even much of our internal deliberation) comes down to what we can express in verbal reports (here "verbal" includes writing, sign language etc). And those reports are derived from the tiny subset of brain-info that gets put into memory in a form that can be processed and re-interpreted as the verbal reports.
If we have conscious experience outside of this, then that experience only contributes to "functional" memory (skills and habits) rather than the any kind of narrative memory. And we never talk about it. I'd say that whatever that experience is, it is a very different kind of consciousness from what we usually talk about. In fact it might be what Freud and folks called the "unconscious".
Usually explanations of technical solutions to problems (e.g. math proofs or code comments) leave most of the actual understanding unstated, and someone trying to understand the solution for themselves has to painstakingly reconstruct their own understanding, which often ends up somewhat different than the author’s.
Many conscious graphical memories and structures (e.g. details of the appearance of someone’s face) are all but impossible to faithfully relate, except sometimes through drawing or the like for expert illustrators.
You're quite right and I'm sorry my comment seened much more "everything is text" than I really wanted.
To me the central thing is not those verbal reports, it is the recording into symbolic memory. The symbols need not be verbal, they can be records of our internal representations of geometry or whatever. But whatever they are, they are "parsable" -- i.e. it's not just neural wiring that makes you able to do stuff like catching a ball, it's stuff that you could re-interpret (imprefectly) into a verbal report.
And you are right, if the thing was a record of geometry, then you are almost always better off drawing a picture than trying to describe it in words.