This is a great question, and certainly one that we know is on a lot of players' minds.
First and foremost, we try to keep Pot Farm as themed as possible to really give it that personality our players love, and part of this means celebrating with 420 plots of land, specifically ;)
Secondly, while Pot Farm is a game designed to let you chill out and relax, we do try to give farmers a chance to exercise their brains with some light strategy! With the space provided, you're encouraged to use hydros for mass planting and storage for items to make the most of your farm, and plan ahead for the next set of quests.
Finally, since we're hosting all the game information on our servers, upgrading the land would mean adding more strain to those servers for our million+ awesome players! We don't want to do anything that would increase the risk that you would have a less-than-smooth experience farming :)