Because he's still too young to know how to put himself to sleep. Kids naturally start learning to put themselves to sleep between 1.5-2 years old (some later, some earlier, every kid is different). Until then they need some help dozing off.
Babies before that generally fall asleep at the boob or the bottle, or some with a dummy.
Self settling is a developmental leap like crawling, walking, etc and will happen when the baby is ready .