Simulation/sandbox games aren't necessarily meant to be "easy" or "difficult" because the nature of the game is generally to explore, experiment, create, etc. Some are meant to teach concepts or specific ideas or tasks, while others are intended for entertainment and outlets for creativity. Difficulty modes don't seem well suited for these kinds of games, in my view. I've never played any simulation game (of any kind) that had such an option, and I doubt I'd ever want to. If the game has a decent foundation I'd expect to work with whatever tools it gives me along with my own imagination. If the game isn't so great at that, I'd simply move on to something else.