Other mistake: During the climax, when Junior reactivates the baby manufacturing machine, the first baby to emerge from the machine is naked. The machine's hands diaper this baby. However, when the other babies emerge from the machine they're already wearing diapers. (01:15:45 - 01:16:25)
Other mistake: When the Stork is hit with a stick, "Perfect!" is heard in his voice. However, the stork had been knocked out when it is heard and we aren't shown who shouted "Perfect!"
Chosen answer: For the same reason people take cars even though they can walk. It's faster and doesn't require using up your energy.
Bishop73