Why are my babies feet flat?

Most are.... The child will develop an arch as they age. Mast babies have flat feet. They start to develop some sort of an arch when the start heel-to-toe walking - somewhere around three years of age.
Arches show up later. As a child grows, the arches in the feet will appear over the years. Feet arches are formed by the bones in the feet, to support the increasing weight of the growing child when the child is standing/walking. If the shape of the bones are such that they don't push against one another to form a stable arch, the bones will drop downwards somewhat, and the foot will seem to have little or no arch.
Totally Normal. The arch doesn't start to develop until the age of 3. So it is totally normal for your baby to have flat feet. As long as the foot is flexible and moves around well at the joints there is no concern. Flat feet can be a hereditary trait so depending on mom and dad, the baby may or may not have a normal arch as he gets older.
Baby Fat. Most infant feet have little visible arch due to the amount of "baby fat". As they thin out and mature, the arch is more visible. Having said that, infants can have developmental deformities of their feet that are best treated early. If you have concerns, take your child to a podiatrist who is comfortable treating young children.
Usually not. Babies have a lot of extra tissue in the arch region that makes the feet appear flat. This usually lessens as they grow and the feet take on a more 'normal' appearance. If you are concerned, however, have your infant evaluated by a specialist that is not only foot and ankle trained by also has an interest in pediatric issues.
This is normal. This is a normal finding in newborns. This does not predict flat feet as an adult.
Immaturity. All babies have flat feet because the muscles, ligaments and tendons that determine the strength of the arch have not yet devloped. In some children, there may never be strong arches, but unless there is foot and leg discomfort on a somewhat regular basis, flatfeet are not a problem. Tha arches develop usually around 2- 3 years old.
It's normal. Baby feet are chubby and flat. The arch does not really develop until about 3 years of age.