ISUs are present, because player allow them that by not building elephants to punish them.
Your logic is similar to 'build raketenwerfer 2 minutes into game to counter WC51'. Raketenwerfer does not properly counter the Dodge per se- it denies a small area of the map. That is all the Raketen does. The Dodge can shuttle BAR rifles and chase squads and do whatever it wants to do elsewhere and the best the Rak can do is creep up and make the Dodge go somewhere else.
The Dodge is the ISU, the Raketen is the Elefant. If the Dodge appears in an avenue the elefant doesn't cover, it can own anything in front of it (save the Jagdtiger). If the Elefant appears in an avenue the ISU isn't? It can't do anything to units with two feet and a rifle. The Elefant can't chase the ISU, and the first real opportunity to actually kill only arrives at vet 1. Worse still- button, one AT gun shot (which the Elefant can't do anything to fight against), an offmap, Mark Target, et cetera can allow the ISU to go head on with it's 'hard counter' and win.
Guess what happens if it wins? You lose everything in range not behind a shotblock- tanks, infantry, crewed weapons. What happens if the Elefant forces the ISU back or god forbid, destroys it? It isn't going to help your infantry push against anything, that's for sure. It becomes a dead weight unless tanks present themselves to it.
And before you say the Elefant can kill other vehicles too, not just the ISU- the ISU does that just as well.