Why are Scaling Quints so God-awful Compared to Everything Else?

well because early game is way more important to have 9 more armor but at lvl 18 27 armor is nothing

if youre a level 6 jungler who plans on not fighting a single time before, then they could be beneficial.

And in aram they are a great deal stronger but who plays/cares about aram?

Depends on which ones you use, they are better in combination with some other runes, but yes, they are lesser in comparison with other Quints, the only ones I find useful are the CDR Scaling Quints.

if you combo them either take 3 quints of scaling and flat ap glphys or flay ad maks dedpends on your champ or do the reverse and take flat ap/ad quints and scaling admarks or ap glphys or if you can do well early and just make it to lvl 6 or 7 which is when scaling is even witht he flat youll be fine