Because we inherited that setup from the first fish to crawl on land millions of years ago. Most of the descendants of that creature have inherited that setup and unless there was a big evolutionary pressure to change things, things don’t change.
Fun fact. That fish that crawled on land inherited his setup from an even older fish which had four nostrils. two of those nostrils however wandered inside the mouth over time.
We still have those two extra internal nostrils too and we use them to allow us to taste things by smelling the inside of our mouth (oversimplification).
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