This is over a hundred year old rule and common in "the new world". You can guess why it is common if you think about the phrase I put in quotes. All these countries are composed of immigrants...

Here's a short and incomplete list: USA, Canada, Mexico, Brazil, Argentina, Chile.

The logic is that the culture is what makes you part of the country, not the blood in your veins.

The other side of that logic is that you're not guaranteed citizenship to the country of your parents. It certainly isn't automatic.

Children integrate into the country so much more easily.