Because now you are facing opponent who doesn't need to build tanks and has plenty of resources for indirect fire and will roll you over silly with superior infantry force, while you're spamming TDs and preparing for heavy tank that'll never come, because now there is no super tank that counters all other tanks who aren't 60 range TDs and bleeds infantry significantly over time.
As USF you get infantry units for teching. Usually you can have a tank and equal number of strong infantry units on the field with lots off useful abilities. If you lost some of them you will lose the game probably because of that, not because of some imbalance, sadly very often players blame units that seal the deal for being too strong rather than realising that some units they lost 5 or 10 minutes earlier cost them the game.
There is nothing that jackson counters that panther can't.
But panther isn't meant to counter tank destroyers, so your only argument you're clinging to is invalidated by principle of balance.
Incorrect. Panther doesn't counter infantry. It is a costly tank destroyer with inferior range and more armour. So, if it can't close on the jackson it will lose. If jackson has no support panther will win. In real game situations range is usually preferred as both sides have lots of support and closing on is more risky due to snares, mines, hand hel at, at guns, flankin mediums etc.
Play allies then?
It won't force you to take off that german uniform for games.
Incorrect. You should suggest that to allied players to realise how difficult it is to play against units with superior range that accelerate more quickly, back off, quickly repair and continue dealing dmg. Their effective health pool is simply higher. I play all factions and have better results as allies, especially with UK. Generally it is easier to play with allies.