They're not flats, they're just comfortable hills as opposed to risky mountains.

Bigger breasts isn't always better; when you wish for a bigger breast, you should be prepared for all the problems it brings. Who would know that behind her facade, Koishi's wishing just that; a convenient, smaller breasts.

And, personally, I prefer flat tracts of land anyway. Easier to till without much problems, unlike hills (let alone mountains), which may require terracing.