Because legalistic people have declared that it's bad to drink and come up with all sorts of un-truths to convince us we should all abstain.
Many people do serve alcohol to their children. It is my opinion (shared by many, I think) that children served alcohol in moderation and on special occasions are actually less likely to abuse it when they reach their "rebellious teenage years." That's because the "forbidden fruit" aspect is missing and because they've been taught how to use it in moderation.
It's similar to teaching children how to use guns. The answer to preventing accidental shootings with children is not to forbid them to touch guns (because then they just get more curious) but to teach them how to properly and safely handle guns and to have the proper amount of respect for them.
It's also similar to teaching children how to eat properly. We don't teach them to be moderate in eating sweets by refusing all candy, cookies, cake, etc, but by teaching them how much is appropriate to eat.
Besides which, the suggesting that scientific studies and facts about the health benefits of moderate alcohol consumption are negated by the fact that uninformed (or misinformed) people don't serve their children alcohol, is a highly illogical form of argument. There are many reasons people do or don't do things with their children that have nothing whatsoever to do with health benefits. I mean, why DO we serve our children so much refined sugar and corn syrup? Those things are categorically un-healthy for growing children. Most people know that, yet they continue to allow their children to eat the stuff... Obviously, parental decisions concerning children's eating and drinking habits often have very little to do with what we actually know about health risks and benefits.