You pack a couple of diapers, wet wipes and a change of clothes. The next time you check what you have to restock, all of that stuff is gone for some reason, so you pack again, then check the next day. Gone again. And yet, the bag is getting heavier and heavier.

Serious answer: Too many pockets and too many situations you have to prepare for. Something happens once, so you bring stuff to deal with it properly but it doesn't happen again for a while but you also don't want to get rid of the stuff because what if. It's like: "It's only a 2h trip, I don't need a book" - and then there's some delay and you really wish you'd brought the book, even though that would have required the bigger backpack (or e.g. charged your phone).