Then why don't you use those "all other units ALREADY pening it" instead of Pz4 or Puma?
Because managing to fuck up so a p4 or puma is in range of your 60 range highly mobile TD shiuld be punished. Kinda like it is for every single other TD in the game, y'know, the ones that don't have all the advantages the Jackson has but also have a weakness to exploit so if you are bad you will lose it. If you can keep it out of range (like you should) it shouldn't be an issue.