Why is the baby wearing a bonnet and nothing else?
It was obviously just so whoever made this booklet could feel proud of him/herself for pairing a bare-bottomed baby with the title "The Bare Facts of Crochet... For Beginners".
What sparkling wit.
Well, either that or the rest of the baby's clothes would be too difficult for beginners to make, so crochet beginners' babies have to make do with bonnets and blankets.
(Incidentally, please observe that the bonnet matches the blanket. This is very important. Babies require matching everything.)