It's not as easy as just taking a 4 wheeler out and spraying. I'm not a range specialist or anything but I understand that some, not all, weed/grass species are receptive to herbicides, some can only be burned, some can only be burned certain times of year, some places the grazing associations and even local residents get real nervous when you start talking controlled burns, some don't want that land taken out of production for that time, some might have ESA protections, etc..
The point is, at least to my understanding (and I could easily be wrong)it's not as simple as pointing a finger and saying go do that today so it's all better tomorrow. The noxious/invasive weeds/grasses/trees is a MASSIVE undertaking, that costs a whole bunch of money, with a whole bunch of stakeholders that have different interests and concerns, and changes are made incrementally...which means small steps over sustained periods. Which also means the business of fighting this is one that can make you feel like you're chasing your tail.