Why no use? We don't know that. If nothing else, the same underground habitats they want for the moon could be build on Venus. Lots more resources. More recent volcanism will have left useful mineral deposits close to the surface we could mole to. There's nothing useful on Mars. Fine dust to clog everything .. an atmosphere that so thin it's useless regardless of that Hollywood might make you think. Even Martian gravity is detrimental to human health. Venus is almost the same as earth.