I really think it's not a matter of lenght but more of repetitivity, gameplay loops etc.
Baldur's Gate 3 for example, while being a very long game, felt so refreshing to me. It's long but everything in the game is unique, even each combat is a specific unique encounter set in the lore, without any grind, any trash mob etc... Each quest is interesting, dialogues don't get boring, and you don't have a map with 1000 icons everywhere, it's up to you to find things by exploring naturally, and if you miss something? That's fine, you won't even know it, you don't have to see everything, because what's great is what you can do, not what you HAVE to do.
On the other hand, 90% of the standard modern open world are now incredibly boring to me. I hate it when I know exactly how these game will play, how they will be structured and all, before even playing them. 20 hours on a ubisoft open world will feel way more bloated and repetitive than 100 hours on Baldur's Gate.
You can also have long games that don't have this classic open world structure. Kingdom Hearts 3 for example with the ReMind expansion can keep you busy for like 90 hours in the critical mode, but it's fun for the whole run because you will never repeat the same task, it's new content during the whole thing.
The lenght isn't the problem, it's more about you (and everyone really, and that's normal) associating the idea of long games with ubisoft style bloat, because that's what is the most common these days and have been for a decade.