If a Gadol came down with dementia or Alzheimer’s or something terrible of that nature, would you no longer call him “Rav…” even though you know he can’t actually function as one anymore?
Of course I would, but it’s an inapt comparison.
Why does someone who spent a couple of years learning certain parts of Yoreh Deah deserve a title of respect, but someone who may have spent more time than that mastering mesechtos in Shas, or other parts of Shulchan Aruch (or even the same parts without a test) not?
The gadol earned that title with decades of toil, and made himself into a living sefer Torah. Luchos v’shivrei luchos munachin ba’aron.