Because you learn that its just another beverage rather than some forbidden fruit.

To go even further, it's just another beverage that tastes nasty. As a kid, my mom and dad let me taste a variety of beers/wines from time to time when I asked, and every time I thought it was disgusting. I still don't drink primarily because even years later I cannot stand the taste of alcohol, even when it's diluted; my secondary reason is because I don't want to limit my experience of life.