It is not always because farmers couldn't produce their own seed, but it is also because growing for viable seeds has a bit different finishing procedure as gro

It is not always because farmers couldn't produce their own seed, but it is also because growing for viable seeds has a bit different finishing procedure as growing food stuff. You might end up with a loss of seed viability because your primary effort is based on selling food for the highest profit and not storing viable seed in the most pristine way. So naturally a local farm community can evolve into a large number of food producers who don't store their own seeds, and a smaller variety of seed farmers to supply the other farmers with crop seed every year.