Meat was not a reliable food-source for our ancestors. Hunting was dangerous and not always successful. Plants don't run, they don't fight back. Our ancestors were likely not so much "hunter-gatherers" as they were "gatherers who occasionally were lucky enough to hunt something succesfully".
Plants were always the primary food source of most omnivores. Humans are no exception to that rule.
Then why do cave paintings depict hunting scenes so often? For the same reason why action-movies are full of expensive sports cars instead of old rust buckets or hand-me-downs.
And better brains don't just make hunting easier, they can also reason better about what plants can be found where and when more efficiently, culminating in the stroke of genius that was stopping to collect plants in the wild and instead mass-growing them in the controlled circumstances of agriculture.