Because the instinct to be stimulated is part of our evolutionary survival strategy. And learning fulfils that instinct.
Kind of like when you someone tells you a joke for the first time and you laugh, you laugh because you learnt something. Say that same joke again maybe a giggle, as you hear keep hearing the same joke it’s no longer funny anymore because you have already “learnt” it. If they’re playing with a stick for hours it’s not stimulating for you because it’s more like a been there done that kind of thing.
Kids and babys can play and laugh at the same things for an extended period of time because everything is still new to them – they’re still in the process of learning how things work, how social interactions are made and we have the mental capacity/intelligence of figuring things out fairly quickly and being done with it.
Kind of like how when people go into a sensory deprivation environment their brains are trying to create stimulation by themselves and they end up going crazy.