There are several answers to the question.
First of all, farmers concentrate their efforts on crops which will do well on their land; if they're on limy clay they grow cabbages, if on sandy silt, then they grow carrots. Few farmers have the conditions to grow a bit of everything.
Secondly, farmers are professionals: they prepare the soil according to scientific formulae and have access to pesticides we can't use, even if we'd want to.
But the third answer may surprise you at first; The stuff in the supermarkets looks even and perfect because a huse proportion of the crop is just discarded. If you threw most of your harvest away, you'd find it much easier to get a basket of good-looking produce, after all.
Some unlovely veg can be sold to soup manufacturers, and the second-best stuff often goes to catering companies or market stalls for a lower price. But far too much is just ploughed in to rot down.
Gleaning - an old idea but a good one!
For more about local gleaning, click here to go to Kent News website, publishers of Kent on Sunday.