Because we're smart as fuck by comparison. Like have you ever seen even a fully grown deer playing Minecraft? A 5 year old can human can play Minecraft. Don't see no horses doing potential differential equations, heck even a pretty immature human can be taught integration. You should see fish trying to assemble iPhones, they absolutely shite at it.

We're social animals with big brains. Other great apes, elephants, whales, even some birds like parrots and corvids mature slower.

We rely less on our instincts and more on what we learn from the previous generations, and that takes time.

Because we're K strategists in the extreme¹. The evolutionary strategy is to adapt and make the best of every offspring by making humans individually adaptive and adapted. That requires a lot of experience and input that doesnt come from a uterus, and that takes time; input bandwidth is limited and it needs to be contextualized, organized.

¹excepting Christians and those culturally influenced by them, who go pretty hard R with it.

Because there is no war in your country. Look at countries where is war and how fast children mature.