Australia has the most fragile topsoil in the world, sometimes thinner than a couple of centimetres (a single inch), so all hooved animals wreak simply too much damage / erosion to be feasible in the elimination of weeds. It's also why feral hooved animals are such a problem. In places where land is over-grazed by stock, salinity goes up, erosion goes up, and it can take decades for the land to recover.

It's strange you know, on the one hand, the plants can survive fires and blistering heat, but it's also a land of immense fragility. Weeds / feral creatres / hooved animals are simply devastating. The fact is that every animal that evolved here naturally have soft feet, not hooved feet, to work in harmony with the fragile environment they were grazing off (I think the exception is the pig-footed bandicoot which, if you google them, were exceptionally fragile themselves).