.why they dont spend money on trying to give the weeds a genetic mutation so they dont grow instead i dont know
Really? Think about it for a second.
You plant the plants that you want to grow. The 'weeds' are already plants in the local environment (or are introduced at a later time, not by the farmer or horticulturists) that can take over and interfere with the plant growth of your choice. You do not plant the weeds! So if you do not plant the weeds, how can you modify them to not grow?
but i wonder if there is another way to deal with agriculture problems...
Cut down more rain forests for farm land, so each farm does not have to be as efficent.
Reduce the population.
Have people waste less food (a huge problem in certain countries).
Do like me, and everybody with a yard have your won garden (the food really is SO damn much better grown fresh).