They know that the numbers are the key ingredients to their earning glowing fitness reports and merit raises. And the various Sector Chiefs are also concerned about promotion so they're conscious of public image. But since there's no official processing involved, the actual headcount is uncertain at best. Lastly, the number of aliens taken into custody, whether officially or unofficially recorded, processed or unprocessed, is in the end completely meaningless from a immigration law enforcement or national security standpoint. Although the technology and manpower are available to do the job, we as a nation lack the will.
Illegal immigration begets more immigration.
The longer it goes on, the harder it will ever be to control. It may already be past that point. Contemplate for a moment the civil disturbances that would likely result if our border were controlled. There are many honorable people who work for the Border Patrol, who do their job in as competent a way as they are allowed, simply follow orders from their respective bosses. It would be counter-productive to your cause or mine to embarrass any of these fine people.