Why glyphs are hard
A letter is a precise shape with a name, a fixed relationship to its neighbours and a meaning. An image generator reproduces local texture and appearance, not symbols with identities.
At small sizes, letter-shaped texture is enough to look convincing, because the eye reads a word as a single pattern. At reading size the same texture is incoherent, and the illusion collapses.
Longer strings fail more than short ones, because every character must be consistent with the others. A four-letter word can survive; a sentence rarely does.
Typography also carries rules: kerning, baselines, stroke weights, case. A model that has learned what text looks like has not learned what type is.