Maybe the odds are just very low? I went and cut literally every edible plant I could find, and planted in an entire field, and always get less seeds than the total plants I started with. The odds of getting the potato equivalent is pretty high, but occasionally I won't get any seeds for blueberries or wheat at all and will be forced to venture out into the world to get more. Even with the farming skill at 25, I always eventually run out of seed.
The odds of getting seed to replant should be an average so that you always end up with at least what you started with, or the crops will eventually deplete in a world completely. Or better yet, maybe the crop itself could be processed to get seed? I mean, a bulb could be split and planted as two. Blueberries can be dried to get multiple seeds. Wheat is a grain so each one should produce an entire plant that will have multiple grain.