Simply because it is a waste of resources. It is impossible to effectively seal our borders. There are thousands of miles of rugged unsecured border with Canada and thousands of miles of coast line not to mention thousands of shipping containers entering our country everyday. Anyone seriously wanting to get in can. We can't win the war here. We have to win it there.
And we have to continue to fight it there, or it will come here. This is the truth of Iraq that the American people do not understand. As long as there is a galvanization point overseas, the al Qaeda efforts will be concentrated there. Hence for the foreseeable future, in fact decades to come, we will have a very proactive military presence in the middle east... regardless of who is the party in power.
Only if 'effectively' by your definitions is 100%. We've effectively built hundreds of thousands of miles of concrete interstate highway and created an electric power grid for this nation; we can build a tough double wall with an electrified inner fence section plus electronically monitor activity.
Will it be 100%, no, but it will sure as hell slow down the criminal activity element.
As one previous post and the article referred to, this is all about CRF, Trilateral Commission, global economy, global government, 'supporting the cause', and most important, Revelations. Don't kid yourself, it's coming.
The one we are losing to Mexico?
We aren't even fighting.