Perhaps this is a stupid question, but I gotta ask, if doing things like this is such a terrible experience and you hate a game, then why do it?
I'll tell you why I've done it before as I've wondered that myself. Part of it is stubbornness. I don't want the game to beat me as it were by the challenge. Then there's the time factor. I feel that if I walk away from it then my current skills at the game may be rusty so if I'm ever going to do it then now is probably the best time while my mind is still well in sync with the game and what its systems and controls are like. There's a little bit of a personal pride or "bragging rights" mixed in with knowing that most players probably won't have finished this challenge but you conquered that mountain. And there's still a sense of some satisfaction that you can mark off and consider the game fully beat and complete. When you look at the game or see its title, you don't think about how you never finished this or that. There's no niggling feeling in your brain about it. That itch has been scratched.
Just to add on to this since Khush touched on some of it, I've always been a completionist with my games, long before there were achievement systems. Just as an example, I have a Wind Waker save file on my GameCube memory card with all the statues unlocked, and you get absolutely nothing for doing that. It probably stems from my family being extremely poor for most of my childhood, so when you got a game you played the **** out of it. You squeezed every last drop of enjoyment and intrigue out of every game you played, because outside of rentals you wouldn't be seeing anything new for a while.
I find that you don't really know what a game has to offer until you've explored all its systems. Sometimes that's for good, like when I beat the original Bioshock on its hardest difficulty with the Vita-Chambers turned off (something that was a lot harder in the PS3 version than it is now in the remasters). That really caused me to experience a game I loved in a whole new way, where I had to really learn the environments and how to manipulate it and the enemies to my advantage.
Sometimes that's for bad, like when a charming little turn-based RPG like Ni No Kuni was absolutely ruined by trying to get all the familiars and their variants. But you don't KNOW going in how these things are going to shake out. You don't KNOW going in that NNK's method of managing familiars is utterly tedious and that some familiars have absolutely ridiculous spawn rates. You don't KNOW going into easy-going World of Final Fantasy how utterly grindy it is to clear all the monster ability boards, or how bullshit the post-game dungeons are.
Going back to Kingdom Hearts, you don't know what an utter clown show Dream Drop Distance's design is until you veer off the critical path. You don't know how the portals are tied into the drop system, because the game NEVER explains how the Forecast system works or how portals rotate between drops so you have to keep swapping characters. I played the entirety of the main campaign preventing the game from swapping characters until I cleared each world with one character. The game ENCOURAGES you to do that, so you don't realize you're actually punishing yourself by skipping content. You also don't realize till late in the game how poorly the Dream Eater system is designed, as the their mechanics are only very loosely tied to the main combat and exploration mechanics.
And you know what? Sometimes grind isn't a bad thing. Melody of Memory was an utter grind killing 100,000 enemies across all the songs, as well as beating all 150 songs on every difficulty and Full-chaining at least 50 of them on every difficulty. Despite that, I found the grind relaxing, because you're really just doing what you do in music games: replaying songs to improve your skills and shoot for higher scores. I found grinding for levels in Yakuza quite relaxing. Just throw on a podcast or movie and run around killing high-value targets, same as players have been doing in games for decades. Just don't actively annoy me while I'm doing it.
One final thing worth noting is that with the way modern games have been designed to appeal to anyone and everyone, there just isn't as much joy in merely rolling credits on a game as there used to be. Modern games roll over. They WANT everyone to complete them. I find achievement systems allow developers to still give their game a bit of teeth for those that want just a little more out of their gaming experience. They push me to explore outside my comfort zone a bit. Within reason, of course.