Why does my baby get such bad rashes?

Maybe sensitive skin. Some babies have much more sensitive skin than others. For those babies, I recommend a mild, unscented baby wash (such as aveeno (oatmeal) baby or cetaphil). If your baby's skin is dry or prone to eczema, try a mild lotion (cetaphil, eucerin, and aveeno (oatmeal) are my favorites). Sometimes more severe rash can be do to a food allergy (even if your are breastfeeding) or an infection, so check with your doctor.
Sensitive. Many babies have sensitive skin. Make sure to use a dye-free, scent-free detergent. Also, a mild soap such as cetaphil of dove for sensitive skin should help. Try aquaphor, eucerin or cetaphil moisurizing lotion. Your baby may have a food sensitivity. Try changing the formula if it is cow's milk. If you are breast-feeding, common culprits are nuts, eggs, soy and citrus in addition to dairy.
Eczema, dry skin, more. I agree completely about the previous advice posted. Babies have very sensitive skin. It is difficult to distinguish between dry skin and eczema. Does your baby scratch, or does the rash only bother you, as a parent. That is to say, is it an itchy rash, or is the skin just red in appearance. Do use mild detergents and soaps (aveeno, cetaphil). Ask your doctor. Use online forums. Post a picture.