Why does water that falls in the Rockies have to flow so damn far to make it to the ocean?
Humans are not the objective, humans are a stop on the road. We just happen to be here, now. The road did not have to pass by here, and there is no reason except random chance that the road passes through humans. No guarantee that we will last for long either.
There is no over-arching "why". there is just random process. No reason (as in purpose), really, that we even exist at all, never mind now, after all that time.