It's not enforced because it's impossible to enforce. In part because it's physically impossible to enforce, and in part because no one really wants to enforce it. For what it's worth, the Obama administration in five years deported more "illegal" aliens that the Bush administration did in eight. You really can't blame the Obama administration for not enforcing the law. He's doing a better job of it than Clinton or Bush did.
I love how both parties are demanding that "something be done" about immigration. Well, end the War on Drugs, dismantle the welfare state, and abolish minimum wage laws. It's that simple.
Onward and upward,
airforce