From a parent perspective, largely because of societal consequences.
Your toddler talking about sex can lead to undesirable social consequences.
Not that I agree with it, but the reasoning is valid, it's a fear of other people and their lack of understanding or nuance. And the potential for them to assume the worst and attack you over something entirely benign.
Now if we're talking about education, there really isn't any good excuse. Maybe it's an extension of the above?
This is what they don't seem to understand. The sex ed is age appropriate. No one is talking about condoms with 6 year olds. They're hearing about boundaries and bathing suit areas. It makes them less likely to be abused and more likely to report abuse.