There may be another explanation. It could be that the world after the zombie apocalypse is different than ours and that due to the atomic bombs, while a harvested crop may still have multiple seeds, perhaps those seeds are most inert or just not viable. It is not unreasonable to suppose that off camera we've planted all the seeds from five crops and only one seedling came up out of all of that which we then transplanted to the planter box. All of that is abstracted into using five crops to craft one seed since this isn't a farm sim game so the devs weren't interested in going into so much depth of having the players act out all the intermediary steps.
At any rate, I think there could be some reasonable explanations that fit with the setting. If by "stupid" you simply meant "I don't like that much scarcity" then that is certainly understandable. But in the sense of whether it could makes sense, I think it certainly can.