There are tests out there that determine if you've had THC lately, but the problem with DUI cases is that you have to prove that you have THC right now and at which amount. THC is stored in the body, even when you are not high and when you are not smoking right this minute. That's the big problem. The only tests accurate enough to determine how much and when take a lot of time and are expensive.
Therefore, they are useless for officers in the field that have to make a determination of someone's sobriety right then and there. Unlike alcohol, THC is absorbed differently in the body. If it got into the blood stream and stayed there for a specific amount of time at a specific metabolic rate, fine. But it doesn't at all.