The objective in balancing is to have the champions play viably when built correctly, it's not to push you away from playing a troll build (yes, that is exactly what it is). These builds aren't 'unplayable' they just suck.
But that being said there are a lot of discrepancies.
AP Varus is 100% viable and works fine.
AD Malz works for split pushing, but is objectively worse then AP because he is an AP champion and therefore balanced around that.
AP Kog is coming back into meta right now, as is Hybrid Corki.
As for the Rest they are pure troll builds, Poppy has one AP scaling on her W, so I don't know in what world you would build her AP for.
As for AD Variants;
AD Soraka is a troll build made popular by that joke of a youtuber, it's not actually viable neither is AD Ahri, which came from a break-the-meta video.
AD TF and Kennen still work fine (infact AD kennen was played in worlds), and AD Nidalee caused a lot of balancing issues in regards to bruiser and tank builds so it's only natural it was canned.
You're complaining about not doing well when you're building your champion wrong.
It's like building full AP Nautilus and wondering why you're not tanky.
I'm not saying don't build your champions this way, but don't complain when they don't do well in comparison to others who are building their champions correctly.
None of these builds were ever viable, they were just cheesy and worked as a result.