I'll take a crack at why Monster Hunter is so good. It's basically a problem solving game.
On it's surface, yes you're fighting monsters in a semi-open world environment over and over again. That's the loop, no doubt about it.
What draws me to Monster Hunter is the epic fights, the difficulty but most of all...it's the preparation. There's just something about min-maxxing a build and seeing it all come together to make what used to be really difficult hunts...well basically less difficult through the use of human engineering and cunning.
What seems to be an impossible fight eventually becomes manageable and later, maybe even trivial. Monster Hunter does a great job at making your progression obvious if you can decipher the systems within the game and conjure a solution to these fights with the plethora of tools that can be at your disposal.
As for the fights themselves, the objective is always the same, but employing your own playstyle, starting with your choice of weapon, the tools you bring to the fight to counter or mitigate the monster's offense or defense and most important of all, your ever growing experience in battling said monster makes you feel powerful. Not just because your "numbers go up", but the efficiency in which you are able solve the problem continuously improves.
It's the planning that works. Being able to use all the information available to you to plan your fights. This is especially great in Wilds. The map system is probably the best I've ever seen. The information is easy to follow. That monster's being diurnal or nocturnal gives the sense that there is a persistent living world the exists outside of your influence, but reacts directly to that same influence. To top it off, the game constantly has the next goal to achieve and it's not about just the next monster that needs to die. You gotta fish or trade for food, mine or trade for resources and help the various communities all with the end goal of preparing you for your next fight.