As others have mentioned, house rules often ruin any fun.
It is, by name, a game about being cutthroat towards other players and emphasizes that you optimize in the long-term purely for personal gain. This actually doesn't come naturally to a lot of people. Even the ones that seem to be more selfish often end up losing because they only optimize for short-term games and make it where nobody will trade with them.
Point is that a lot of those house rules slow down the main driver for the game: forcing people to go bankrupt. The house rules often put money back into the economy, which prolongs bankruptcy. To a large degree, the faster you can speed up bankruptcy, the more fun it is.
One of my personal favorite additions to the game was the Speed Die included in some versions. In a pretty elegant way, it greatly speeds up both the property acquisition and bankruptcy phases of the game.
Also as others are pointing out, the fun a of a board game is 90% dictated by the people you are playing with. That's not to say Monopoly is inherently or intuitively conducive to keeping the experience enjoyable, but it also can't completely prevent you from having fun. As evidenced by plenty of people, myself included, having immense enjoyment from it with the right group.