One plant can't be super-efficient at everything.
Think of it in terms of where the plant is spending its resources. It is gaining energy from the sun (photosynthesis) and taking in carbon dioxide, water, oxygen and nutrients from the air, ground water and soil. It then has to allocate those resources to doing various things. For instance:
- Get bigger to become mature and be ready to reproduce.
- Get bigger to compete with rival plants for sunlight (throw shade on the rivals).
- Protection from predators and disease - spines, toxins, tough indigestible structures like wood.
- Store food for times of hardship. This is why plants have tubers.
- Make structures to conceive the next generation - pollen and stamens, flowers, catkins and so on.
- Grow the next generation (nuts, seeds) and the methods of getting them dispersed (fruit, thistledown), or protect them from harm (hard shells on nuts).
Basically your plant has to choose what it invests in. If you want it to be a jack of all trades (a generalist), which can do all of the above for a farmer's benefit, then it will be mediocre at all of them. In contrast a plant which has specialised in one thing will be vastly better than the generalist.
Potatoes are fantastic at producing tubers - let's say they put 80% of their energy into tubers and use the other 20% for growth, disease resistance and so on. Strawberry plants are fantastic at producing fruit and seeds - they put 80% of their energy into fruit. A plant which tries to do both will have to split that 80% into small tubers (40%) and small fruit (40%). So it will not be as good to a farmer as either a potato or a strawberry.
Another thing to consider is how the farmer grows the plant. Flax plants (Linum usitatissimum) can be grown to produce fibre for the textile industry (for weaving canvas and linen). Or they can be grown to produce seeds for crushing to produce linseed oil or grinding into flour.
If you want to produce fibre, you grow the flax plants very densely packed together so they grow tall and straight. Harvest early for fine fibres to weave linen, and harvest later for coarse fibres to make canvas for sailcloth or rope. If you want seeds/oil, you grow the flax more spaced out.
Another thing to consider is monoculture and crop diseases. If your farmers rely too much on one crop, then a disease outbreak will be devastating. If you grow multiple crops, then the fact that all your carrots have died of carrot-blight will not affect your wheat or your apple orchard.