Achievements do not have any value if someone can "cheat" their way to them. Mods can very simply make the game to easy, reducing the value of achievements. That is probably the reason why.
Now... is it worth it? I don't think the people cheating to get achievements mind, and it's not like people compare achievements. It's more of a vanity thing for many people I imagine, but those that do care about them can't enjoy both that aspect of the game and its mods.
