The reason is that CA is an ability you will always have access to if you have a Pershing (Barring edge cases, such as if you recover an abandoned Pershing, but that's obviously not relevant), and it directly benefits the unit in question. The ability is perfectly able to be considered when speaking of the unit due to accessing the Pershing being strictly predicated on access to CA.
If, for the sake of argument, the only OKW (assume OST don't have a Tiger I) doc that had a Tiger I was Elite Armoured, then if you were talking about the AT capability of the Pershing vs the Tiger, you would absolutely be right in factoring HEAT shells into the argument.
There are differences. HEAT you activate and that's it.
CA requires you to blob or semi blob to be effective. Considering the population, map design and anti-blob werfer/stuka, CA is seldom effective. HEAT is always. You either blob around Pershing/Jackson and risk losing everything (good luck popping CA on maps like ettelbruck, angermunde and the likes) or you send in one or two squads around the Pershing/Jackson and basically don't get the 120 munitions worth. Jackson does nothing to infantry. Pershing is mediocre albeit good compared to P4 and the likes... so yeah, CA easily countered by infantry and if you have 2x raketen or paks supporting it, well CA is wasted. Heavy cav has no indirects besides pak howi/scott and if those are in the game, then I doubt you have the pop for 2 tanks, Pershing is your only tank.
Is taking CA into account needed? Yes
Is there a lot of BS about this ability? Yes. People take it like it's got no downsides.
B) Requires infantry and to really be worth 120 muni, requires you to blob
C) Teamgames map design is annoyingly bad and favors stuka/werfer for blob control, not to mention the lane-y nature. This all reduces effectiveness of the CA ability
I dare people to play Heavy cav on 3v3 maps and use CA. 80% of my games are heavy cav, 20% are recon and each and the only time I've managed to effectively use this ability was against braindead players.