That is because you choose to compare it with medium tanks while it is a TD. I has less range then most allied TD, less penetration and less accuracy. On top of that it cost more and has higher pop.
PzIV is good vs mediums but it is not got vs premium mediums and get rolf stop by heavy tanks(not super heavy).
Allied TDs get 3 times the bonus Panther gets while more accurate to begin with and while vetting faster.
The panther has the best aspects of mediums and good aspects of heavies, such as armour hp and heavy crush. And its range acc and pen are in between mediums and td's. Its only 2 real minusses are low ai output and its justifiably high cost. Its not a td its at focused tank.
The p4's are better vs stock allied mediums in at and better in ai the 2 out of the 3. They should not also beat premium meds of allies. That what you have stugs jp4 for.
The panther is meant to be beefy and quite mobile as its base stats show. Allied td's are meant to outrange and snipe away at axis armour. Both their vet show this. And yet somehow the panther needs vet that is apropriate for td's while allied td's dont get vet that is apropriate for panthers.
The acc buff on the panther is illogical. Axis have plenty at for every stage of the game. Acc is less imporatant vs heavy and super tanks. This is where the panthers time frame lies, big tanks are easier to hit.