This whole idea of an "anchor baby" is patently absurd. Under US law, children born US citizens are not allowed to sponsor the immigration of relatives until they turn 21. So at the very minimum, this alleged plan requires people to wait 21 years and 9 months to legally enter the US, from the time they decide to try to get pregnant.
The reason immigrants want to have children in the United States is for the sake of the child, pure and simple. They love their kid and want it to have a better life at some point. Many illegal immigrants who have a child in the US wind up getting deported and taking the child with them, but they at least know that in the future the child can return to the US and claim the benefits of US citizenship, usually after they have grown up.