Because it has the potential of reserving nearly all of your leisure time over months-years. Caring for a baby while a crazy 3 year old wants to play/eat/fight can be overwhelming compared to just one kid. There's also the organizational side - every step outside the door becomes an endeavour. Then there is housing - good luck with that 100 sq feet apartment (it could work with one kid, but impossible with two). Same with small cars - forget them with a 2nd kid.