Maybe the womp rats were feral. Introduced species now sucking up all the moisture and eating what grains were eked out of the soil. Real Men shoot feral, non-native massive pigs from helicopters in Australia, they lean out of their manly helicopters and fire mortars at the things in the wetlands and they blow up. No one cares because the pigs are trashing the environment and delicate marsupials are dying because of them. You can't even eat these pigs, they are full of worms.
So maybe this was the case with the womp rats, perhaps there was even a bounty paid for them to rid Tatooine of the vermin. Luke might be an environmental hero.