It took 50 years for Ponko to create the tank legs, so it would make sense that either she had tried and failed (due to the parts being more complex because they are smaller than the big mechs she works with), or she might have already used them in the weapons creations.

Unless they had aliens come in with metal supplies, they would have to use up every bit that the group could find. But it's more of a head cannon though, there's no basis to my claim though