Just because you don't see it doesn't mean it doesn't happen.
I'm going to assume that a relatively little population wants to buy/play a game to hack it. Most people would rather just play the game...
Hackers usually want to show off to other people or to ruin other players' experiences. There are steps you can take to mitigate this. Two common targets are leaderboards and multiplayer.
1 - Reality check for leaderboard submittal, or culling of obviously doctored submissions. Friends-only leaderboards works well too.
2 - Authoritative host for multiplayer.
For a single player game where hacking will not affect the experience of anyone other than the hacker... don't worry about it, just let them do their thing. They're having fun their own way with your game, and you should be happy that they are enjoying your game.
Regarding secrets... ever since the internet happened there are no secrets anymore for those who look for them. RIP.
For achievements, either use some sort of reality check or just don't worry about it. Really isn't a big deal.