You never know! Tomorrow that bridge might be on the moon, and how on earth (or how on the moon, indeed) will you know if it will hold up when you walk your herd of elephants over it!
I mean, that's still fine? The max load of that bridge on the moon will just be a different, higher value in kilograms. Since we don't move bridges between planets, it's fine to not advertise a universal value that applies everywhere.
