They probably also just throw some hurdles in the way to make it harder and so are still hackable, unless they have a full server-side approach. Having a server-side gameplay system is extremely complicated and would often require rearchitecting the entire game, as it works more like a multiplayer game (with the server being a host and the player just acting as a peer in a remotely-hosted game). That could well be more effort than making the game in the first place, so it can be difficult to justify.
I'd say: who cares? If some people hack the game, let them. If you have a global hi-score board, yep, it'll get hacked. Often games just show scores for you and your friends, or other people with similar scores, rather than just the top scores, so obviously fake top scores never appear anyway. You can also moderate your scoreboard so obviously fake scores (e.g. 9999999999) get deleted.