Because it's what both men and women want in heterosexual normative relationships. Any man that has tried to seriously date women older than him knows that.

For the exact reasons as to why that is, I don't know exactly, I'm not an expert in human behaviour and societal norms. One could guess that women value the experience, maturity and status of a man a few years older than them, they are usually further along with their journey though life. Men could value youth and perceived innocence of someone younger than them. But it's something that's prevalent in pretty much every society accross the globe, so it's something to do with general human behaviour.

I talking about small age gaps of around a few years here, not the large ones.