Because it shows reverance to God and that we desire to speak only to him for the joy and pleasantness and privateness of one on one with God. Just like when we want to be alone with someone we love that is here on earth. We are showing God the same desire, the same love and he is who we should be showing it to more than that special someone on this earth. Also to pray in front of fellow man just for the sake of doing so and caring about what they think is not reverance and respect for God

Prayers in secrets are more sincere because one is free to tell God everything without shame or fear.