Why do my Farm Upgrades Keep Disappearing?

Edit: Okay so I saw another thread where the question was answered and it seems like it's a feature you only get the benefit on good harvests. But this seems really dumb, because farm upgrades wouldn't only apply on good seasons and in any case it makes them pointless, you get maybe +100 gold and .5% population bonus 1/4 of the time? This makes the investment far below the average "interest rate", so to speak. I could build more troops to get another settlement, build a philosophy tower to get 250+ more gold per turn every turn, or build a market or library to get at least 150.