The duty of a religion is to guide humanity to uphold certain noble principles in order to lead a peaceful life and to maintain human dignity. Otherwise it would be impossible for us to claim superiority as humans, for we would be relegated to the level of other living beings whose only purpose is to obtain food, shelter and sex. If human beings too spend their lives only to satisfy these basic primal needs, then there would be nothing much to show for ourselves as humans. Humans have transcended mere survival and are capable of seeking self actualization.
In Buddhism we call this Dharma. Other living beings cannot realise this Dharma because human intelligence is superior to that of all the other living beings in the universe. Only the human mind can appreciate the Dharma. It is significant to note that humans are the only living beings in this universe who can conceive a system as complex as religion. Even devas and brahmas have no particular religion.
Although we worship devas or brahmas and do some offerings in their name, we must realise that our human intelligence is superior to theirs. That is why a deva or a brahma cannot become a Buddha. Only a human being can attain supreme enlightenment because only he has the capability to develop his intelligence to the highest level.