I totally get how you don’t think about it. What you were told worked fine enough. The only reason I thought about my gender was the dysphoria. Someone who maybe they’d be happier if they were a bit more androgynous or something, or someone who might be interested in dating a member of their own gender of the opportunity presented itself can very easily just not notice into adulthood if they have no reason to, and for some of them noticing comes with a nagging feeling to act on it. Suddenly a person who had been more or less content as a guy who just doesn’t engage in any of that macho stuff, may feel the desire to add feminine touches to his wardrobe and use he/they pronouns. Or a guy who had been perfectly content thinking of himself as entirely straight actually starts noticing when a guy is his type and now has to worry about judgment from partners and family regarding his bisexuality.

I agree with you that all healthy adults should regularly be engaging in self reflection of all sorts. But also, self reflection is painful and difficult. I wouldn’t be the least bit surprised if a significant portion of adults just block out those thoughts every time they arise, at least given how people act.