1) They need so much stuff! Bottles, diapers, toys, books, clothes, art supplies.... we could all go on forever.
2) They keep growing! Just when your kid has grown into one size, the seasons change and you need to buy a whole new wardrobe. Just so it all won't fit next year.
3) They're messy! (Usually). Kids don't know it's gross to eat off the floor. They also don't know that stepping on Legos is horribly painful.
4) TOYS. Toys = stature on the playground. The more toys you have, the more other kids want to play with you and said toys. And, then they get older and want new, different toys. But, they don't want to get rid of all the old toys, either.
5) You're sentimental! It's your baby. Of course you want to keep every single piece of genius artwork and paper written. Of course.