Why do weeds grow almost anywhere so easily, but growing food crop requires so much care and is so difficult?
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A weed is just a plant growing where you don’t want it.
When you prepare a field you create an ecological vacuum with ideal growing conditions. You want a plant bred for its usefulness to humans to grow unimpeded and without competition. That’s the work you put in: clearing out everything else so that only the desired plants will be present.
You’ve also created an ideal space for everything else to grow, and plants that can get their seeds in there most easily will do so. That’s just what they do, and is the reason why clear fertile spaces devoid of competition are rare and short-lived in nature. To be clear, crop plants will do this as well if they can (they are bred for other things do are less competitive in this regard). I’ve seen wheat and corn growing on the edges of fields long after that crop was grown there deliberately. Were it virulent enough the farmer would have to put effort into fighting back prior crops too. That would be good reason not to breed such traits into them, but this would probably not be an issue anyway.
Vast monocultures see whole fields of a single species with the same nutrient requirements. This drains such nutrients from the soil and inhibits future growth, so you are unlikely to see last year’s weed crops out competing this year’s crops. The traditional solution is crop rotation: to plant different crops in a given plot each year, that rely on different nutrients and in some cases put them back. Such rotation usually includes a fallow year, where a field is left unsown and nature is allowed to reclaim it (or fallow seeding occurs in which non-crop plants are sown). This restores the soil for the next crop.
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