I think this is an absolutely great game and the devotion people have put into completing it, despite it being nearly-impossible, demonstrates this.
Making a game fun is a challenge and it takes more than just making it difficult. In fact, for many people, a ridiculously difficult game is the opposite of fun. But IWBTG makes a game out of the difficulty, and that's what makes it fun.
IWBTG is intended to be ridiculously hard. The player is aware of this. They expect to die. A lot. In fact, the point of the game isn't to save the princess or see the next cutscene, but just to be able to say you've finished it. Winning the game is serious bragging rights and that's part of what makes it so addictive.
But it's more than just hard. It's a good challenge. The first level demonstrates this beautifully. The first time the spike trap hits you, you know this is a ridiculously hard game. Eventually you figure out the trick. The next spike trap is similar, and you get that feeling that you've bested the game. Then the third spike trap throws you a loop and hits you from the other side. Not only is it unexpected, but it requires an entirely different approach -- it's a new problem. This is what makes games interesting: a continual stream of new, interesting challenges.
To add to the fun factor, the game constantly leaves the player wondering what is going to happen next. From objects flying upwards instead of dropping down, to the moon falling out of the sky and attempting to crush the player, to fighting a gigantic Mike Tyson, there's always a new surprise, tempting you to see what each level holds. You want to complete the level just so you can see what the next one will throw at you.
And, most importantly, despite the difficulty, the game is never unfair. Sure, it requires split-second precision, pixel perfect movements, and even requires some intense memorization, but all of this can be overcome with practice. It isn't random, so the player never has to leave things to chance, and it doesn't kill the player for unknown reasons, so the player always knows what the next obstacle is and can reason about how to overcome it.
From the challenging levels, to the wide variety of challenges in each level, to the mocking, self-referential humor, to the nostalgic artwork, this game does everything well. So well that people are willing to drive themselves to violence trying to beat it. And others are willing to follow suit, knowing how frustrating it will be.