Most fruits & vegetables are domesticated & need rather specific conditions to give a good result. Weed is adapted to the local environment & hence has already the specific conditions it needs. (Or better: You see only the weed grow which is adapted to the conditions in your garden. Weed which is not adapted to these conditions doesn't grow at all or is overgrown by weed better suited to the environment.)
That makes sense, I've never thought of that, but I'm always angry when my plants don't survive and weed does. Thanks for explaining this to me.