Why aren't seeds available in the store?
From a perspective of how the real world works this makes sense-- people buy seeds and then grow things.
From an in-game perspective it also makes sense. Seeds hide on the map pretty well. They're hard to tell apart. When you want a particular one it's nowhere to be found. The process of finding seeds is just frustrating, and it doesn't really add much to gameplay.
I would gladly fork over a few coins to get the seed packet that I need rather than waste my time scouring a map hunting for the seed packet in a haystack. Take my money, please?