why is there no warrenty

Longevity depends on how you install it and what is the environment. For example, contact with certain metals like lead or copper, or exposure to acid rain will cause quick corrosion of galvalume. Because they have no clue about what you are going to do with the material, and where and how you are going to use it, they cannot give a comprehensive guarantee. It is too much to ask for a guarantee on something that is almost a giveaway price-wise. If you install it right, this material will have longer life span than shingles.