Because the issue is more complex than you realize.
Lets imagine that there are two farmers next to each other both growing wheat. Farmer A is "organic" certified and doesn't use GMO modified wheat. Farmer B does use GMO to maximize yield and thus profits. Both Farmers sell their wheat to two different food processing companies, who then process the wheat and sell it to a different food distribution companies, who then sells the processed wheat to end consumers.
One such consumer is a small independently owned bakery in Vermont who sells packaged bread to other local vendors. This bakery decides to sell only healthy organic, non GMO foods and purchases their food ingredients to conform to this.
Now here is the "crunch" question.
Farmer A finds out that his crop of wheat were cross pollinated from Farmer B's fields when Monsanto comes by, tests their wheat, and claims a patent violation by using unlicensed GMO wheat strains.
The Food Processing company that Farmer A sold his, what he believed was non-GMO "organic", wheat to has all of their food products now cross-contaminated with GMO food products.
The Food Distribution company that Farmer A's wheat is sold to is now selling food that "may" contain GMO without proper labeling.
The small Independently Owned Bakery that bought Farmer A's wheat is now selling baked goods that were supposed to be "organic" but now "may" contain GMO food products.
Who is legally culpable for violating the law? Who is legally responsible for any damages (re: lawsuit)?
Bonus question: What if one of the suppliers in the product distribution chain knowingly omitted labeling foods as containing GMO's to attempt to cash in on the "organic food craze" and make a quick buck by defrauding consumers. Are their consumers now legally culpable and responsible?