The answer is more of “prove it otherwise.” Why would animals not have a consciousness? Consciousness is the main ingredient for life. It animates the energy of whatever or whoever comes into existence. And energy is everything, including the one that we consider life or not. Earth has a consciousness, trees have a consciousness and so on.

We have decided that animals do not have a consciousness so we can kill and eat them without a dilemma. It is not the same to kill a man or a sheep, we may say, so we do not feel guilty when we do that to animals. It is just for our convenience.

If we have ever had a pet, a dog or a cat or even fish, we will come to the realization the animals have a consciousness. They are one with us. In some cases, they are better than us regarding awareness. They love us unconditionally. We may hit them, we may push them, we may yell at them, but they will never stop loving us. Again and again, they will come to us with love and humility.
Animals not just have consciousness, but they are great teachers here on Earth.
Love them the same way they love us, consciousness to consciousness.