They simply can't. The best that can be done is about 6% for photosynthetic bacteria, who don't need to waste energy making cell walls, roots, and other defining features of plants. Genetically modified bacteria have been made that emit ethanol and diesel molecules, but that only becomes competitive at about $100/barrel for petroleum. Prices haven't been high enough for long enough to get that industry off the ground, and attempts to make the process cheaper have stalled.