I believe that it's mostly because you need to abstract away from the visual form, be it a picture or text, and that, for complex programs, it's easier to abstr

I believe that it's mostly because you need to abstract away from the visual form, be it a picture or text, and that, for complex programs, it's easier to abstract away from text than a picture (being more dense may help, but I suspect it's more than that - the apparent visual structure may be a distraction).