Putting on my early neolithic archaeologist hat, there are a lot of things that don't show up in art. The trend in the past decade has been that better methods and new inquiries are turning up even more evidence for early broad-spectrum plant use everywhere we look.
1) Cave paintings may be related to religion. Hunting is fickle, you need to pray to the gods -- show them pictures of what you're after -- to get their help. Bread, baked from seeds gathered from plants, is more reliable; plants don't run away. So you don't need to paint any entreaties to the gods on how to help you get it.
2) Hunting, cave paintings and religion may have been related to one group of people, seed-gathering and baking to another. It may just not have occurred to the painters to include the activities and products of the gatherer side of the hunter-gatherer economy in their paintings.
