It involves adding in lots of details, yes, but adding in completely irrelevant details that aren't related to the main plot is still a bad practice. Short stories you cut out all the stuff that isn't [i]completely[/i] necessary to the point you're trying to make; novels you don't. This is what I mean by 'a little bit sloppy'; sure, some of the details you might be inserting could be cut without completely wrecking the reader's ability to understand what's happening, but you leave it in anyway to flesh out the world or the plot or the characters a little more and to keep the pace nice and even.
I think we disagree on what's even somewhat necessary. Yes, I'm aware that what someone Wants to do is to establish character or world build, but authors regularly do too much of it when it's not only not needed, but not important or interesting or insightful at all. I can pick up any novel and find at least 30 pages of stuff that serves [b]no[/b] concrete purpose. Many times I will find myself looking at a scene and going "What was the point of that? That added nothing." The same for a few subplots I can think of and at least one furry book I would characterize the majority of the actual book accomplishing absolutely nothing.
Typically, the above is when either character actions accomplish nothing (do not move forwards, do not create setbacks, do not gain new information or resources), repeatedly demonstrate previously established character traits or repeat world building that’s been the same (look, a new town that’s exactly like the last one, with people doing the same thing already seen), or adding things that either aren’t interesting at all or don’t matter. I don’t care if the protagonist doesn’t like toast, or failed to get into little league, if these things aren’t relevant nor do they give me any actual insight into this character.
I think it’s erroneous to think you need full scenes and chapters to world build or provide characterization. A lot of these things can be done with a very small word count, slid into scenes beside more relevant things. It’s the difference between looking out the window as the car is moving versus stopping the car to focus on the landscape. Or, if you need full scenes and chapters, then the obvious solution is to actually make these points plot relevant, so you have a reason rather than just wanting to show off the setting’s window dressing.
Let’s take characterization. Characterization is almost all motivation and actions. We should know the character’s motivations based on the events of the plot and the character’s words/actions. We also need to know why the character is motivated to do what they are doing - we likely will get this from their motivation directly (the duke killed his wife, which we see on screen), or plot relevant background information. “Wait what, characterization is all about personality.” Personality is demonstrated through action and dialogue, which you can do on every single page. You can demonstrate it as the character moves through the plot. Finally there’s background, which can be easily summed up in two lines of dialogue and/or tied to other plot relevant points. There, every single bit of characterization, either executed leanly and with few words, or tied directly to everything plot related and therefore absolutely necessary.
Also an evenly paced book is a bad move. If it’s all the same speed, there’s no tension and no points of relaxation. If the book is the same pace - let’s say, all tense excitement all the time - then it’s desensitizing the reader. Too slow and there’s no excitement at all, and it’s a sluggish slog. By changing the pace, you give breathing room and contrast. A varied pace is important, not an even one.
One of the reasons, I think, that very few people read short stories on a regular basis (and this is a fact, last I checked; novels are far more popular than short story collections or literary magazines) is because novels have this nice, slow pace that lets the reader get to know the characters and the world, build up a relationship with everybody they're reading about, get expectations about what's going to happen next, and so on and so forth, whereas in short stories it's more just about reading it, getting the point, and then taking away the message.
Characters aren't important to me; it is rare that I'm actually interested in a character and want to know more, and regularly I'm told too much about every facet of their existence than I [i]want or need to know[/i]. I care about plot. Novels have more things [i]happening[/i] than short stories. And the things that happen in novels - large-scale events, epic climaxes, huge confrontations and big fat spectacle - can't be attained with short stories.
IMO the problem of short stories is that you don’t know what they’re about until after you’ve read a few pages - even with an anthology, all you have to go on is genre and perhaps a theme. Compare this to picking up any novel and seeing the description on the back which tells you who the characters are, what the conflict is and generally what’s at stake. Not only do short stories not have that*, but a blurb might even ruin some stories to begin with because they’re all about the surprise concept in such a small space. It’s a matter of expectation. This is why well established short story series - Conan, Sherlock Holmes - do well. These stories don’t give more characterization, you don’t learn more about the titular characters, but you have a solid expectation of what you’ll get before you start reading even if you don’t know the details.
Also, and this might sound silly, it’s easier to tell a sucky novel over a sucky short story. A short story, you don’t realize it sucks until maybe half way through, and then it’s almost over and you can hold out a little hope - then boom, it’s over. So I just read something that wasn’t good and I’m not happy. A novel? You can generally tell in the first chapter or three, and by then you’ve only read maybe 5% of the book. I feel much more comfortable chucking a novel after that point than I do abandoning a short story in the middle.
*There are some places where they do (the blurb on the back of an anthology, an ebook that’s just that short story) and when they do, it makes me happy.