OST faction get to have only a single stock casemate TD, because panthers are more like a heavy tank with good AT rather a heavy TD.
If panthers were Heavy TDs as jacksons are, they could reliably destroy another heavies like KVs, Churchs, IS2, ISU, Comets.
*Spoiler alert: Panthers cant cost effectively win against those heavies, as we all know, they trade hits as much and can loose miserably when the allied player has 3 functional neurons, axis can only spam panthers to get an advantage. So it cant be considered a heavy TD. At most an hybrid of heavy+medium TD
USF also have ATG, handheld zooks and some doctrinal variations to compensate AT power, like OST does with Elephant and OKW with JT.
And before people get their pitchforks and start a riot, soviet conscript dont have stock weapon upgrades and thats ok, now usf has great doctrinal AT units but they never appear in the discussion of jackOPns. GG then...
Because ostheer units aren't designed to overlap. Each one of them accomplishes a unique task on the battlefield. Paks can handle all kinds of armor and has no problems against allied heavies. Stugs counter lights and mediums and can zone out heavies with a bit of luck. Both of these units also have access to TWP. Panthers are a dominance tank: Can help units against infantry with its 3 mgs, can realiably penetrate mediums and has the speed and armor to dive in finish off damaged vehicles.
Not even soviet units overlap that much. It follows the same reasoning: Expensive Manpower only ATG that can handle even heavies and T3 tank hunters that exchange penetration and range for mobility and the lack of a set up time.
You can't compare "the variety of usf light AT solutions" vs axis doctrinal heavy TDs designed to zone out mediums and kill heavies. They serve different pruposes, have different timings and costs.