Man, I remember how much FUN cheat codes could be.
Big Head, DK mode, Paintball, infinite dual-wielding rocket launchers, and invincibility in Goldeneye? Awesome!
All weapons, level select, blood color choices, Wireframe mode in Turok? Amazing!
Access the Sonic Debug Mode? Sweet! Perform the Chrono Cross secret room disc swap? Cool! NARPAS SWORD in Metroid? Only way I could beat it the first few times.
I miss cheats. I miss cheat devices. Even if it wasn't "cheating", I loved making Mario grow and shrink in Mario 64, giving Link a "Gold" Tunic in Ocarina of Time, and using an Action Replay to unlock more than half of F-Zero GX (because you needed to transfer your memory card from an F-Zero AX machine... and only 7 exist! Screw that!)
I totally exploited glitches in Pokemon to fight MissingNo and duplicate Rare Candies. I loved using a Gameshark to get to the "mysterious island" in Goldeneye. I loved using a cheat device to see hidden, creepy, disturbing, sealed off rooms in Silent Hill 3. I most definitely enjoyed using them on long-winded Japanese RPGs to reduce level grinding and on Nintendo-hard impossible games like Battletoads. Man, I really appreciated that "There Is No Cow Level" cheat in Diablo 2 and "Power Overwhelming" helped me through Starcraft.
I miss that era a lot and it's a shame so few keep that spirit alive.
The only game in recent memory with "cheat" codes was Mortal Kombat, with the Kombat Codes that modify matches (Zombie Combat! Rainbow Combat! Vertigo Combat!), but that's pretty much it.
It's the combination of not JUST achievements (though that's a big reason), but also how the online community has thrived and fairness in games like Halo or Call of Duty would go out the window with cheats (not that modders don't do it anyway). If they could bring them back, I'd be all over them.