Sex education does not just mean teaching the biology of sex, which is instinctual. It deals with issues of consent, social norms, disease prevention and healthy living.
It is true that humans have survived for millennia without formal sexual instruction. But sexually transmitted diseases were rife, sexual assault has been common and as a race, we have been historically intolerant of the LGBTQ+ community.
Sex education can help with all of these things.
Just because humans have survived for millennia, doesn't mean that we can't aspire to be better. Sex education is part of that aspiration to create a better society for everyone.