The US does in fact have a pedestal ground mount for Hellfire and 70mm laser rockets, it can fit on a hummve but only a handful were ever built. A tank mount makes no sense, the 120mm gun is far more economical for precision long range firepower if your going to bother to pay for such a big vehicle.
TOS-1 was a replacement for the OT-55 in Army and Front level flamethrower tank battalions, rare units intended specifically to assault heavily fortified areas. Same way the RPO rocket launcher replaced normal manpack flamethrowers in sapper units. Since a Soviet Army already had a good fraction as many tanks as the entire active duty US Army does today, were talking about a really niche weapon in scale. Only the shear size of the Soviet military justified producing it, as niche per army adds up when you have several dozen armies. The area bombardment rockets are mounted on a tank because they are big, heavy and yet very short ranged to allow the largest possible warhead. So the tank hull is actually a requirement to get it into action with a reasonable chance of surviving. This is not true at all of a weapon like Hellfire which can fire off anything, has a much lower launch signature and only engages point targets.
The US Army solution to heavily fortified areas is GMLRS. No reasonable field fortification or urban structure can withstand attack by that weapon and at 109,000 USD it only costs about 50% more then a single Hellfire. With ten times the warhead. No good reason exists to build a multiple launch rocket system with incredibly short range. The present intention is to build 100,000 of them and they are being produced at the rate of some ~3000 a year. Russia seems to well understand the advantages of more flexible modern weapons, as only a small number of TOS-1 vehicles seem to have ever been produced.
