The simple reason is people who drink and drive don't show.... You have a very quick intervention as a police officer, and if you don't have the overt signs of intoxication right there, you're not picking that up. A lot of times at .05%, you don't see those signs in that quick interaction because, in a sobriety checkpoint, you're moving people through really quickly.
It's interesting when you look at arrest rates. When you look at how people are arrested, 70% come because of random police patrols where police have observed some divergence from regular driving, 24% come from where they crashed themselves, and 6% come from sobriety checkpoints. Again, obviously observation is the key method and the ability to test them right away is very effective.