I think i know why! I think its because zen garden plant drops have different plants than mary golds(wich yes must be rare) and chocolate is not useful at all (and you all know its for the snail to wake up and get speedy)
On certian versions, when you feed chocolate to Stinky and exit the Zen Garden, when you return, it will say "While you were away Stinky collected _____ coins (before falling asleep)!" (Parentheses mean that it is only there sometimes) Those coins then get added to your wallet. Also happens without chocolate. I have 9 plants and get 4k+ cash for every chocolate I feed to Stinky.