For one reason, because CONGRESS controls the purse strings. I'm sure there are others, like going through channels to make laws so they can be inforced rather than by fiat.
Your frustration is shared. Our borders must be better protected.
That said, the Armed Forces (including the National Guard) are not the right assets. They already have missions and are not sitting around waiting for something to do.
The National Guard is a particularly poor candidate for this type of service. It is not a federal asset. The National Guard is the organized portion of the states' militias and commanded by each state's governor, not the president.
The National Guard units are further structured around integration with the regular armed forces for call-up during times of crises. This federal call-up is only for a limited duration.
What is more appropriate is using the assets whose mission is protection of our borders. Namely; Coast Guard, Customs, Border Patrol, INS, etc. And GWB is doing something about that. It's called the Homeland Security Department.
So instead of trying to waste our armed forces with totally inappropriate tasking, we should climb on our congress-critters' backs and whip them into taking the shackles off these appropriate organizations.