How many nappies do you think the average bundle of joy gets through in an almost 24 hour period? You also need their baby food, a changing matt, baby wipes, nappy rash cream, clothing, books and toys to keep them entertained.
Last time I travelled there was hardly room for a small bag under the seat in front, there's no way 24 hours of supplies for a baby would fit easily. Now what if you have twins?