In a short story you get to tell it like you were talking to a frined, or gossiping about a story you heard. It's over pretty quickly, so you really don't have to flesh out the characters as you do in a novel: they appear, they act, they get some challenge, and they overcome it, and then it is the end. There may be an antagonist or two, but you really cannot clutter it with too many people and threads of plots, as you do not have time or space to proliferate.
In a poem, you are attempting an emotion or maybe a few emotions, but it is not a story. Yes, it takes a certain amount of talent to write good poetry, all the other poets (who consider themselves poets) are good for teaching poetry to K-12 students, but the really good poetry is left for those very special human beings that feel and express high above the usual masses. There are different subgenres of poetry as well, and it takes a number of talented characteristics to pursue such subs as well, for instance: epic poetry vs. haiku.
Then, you have chapter books, which were originally created to engage younger readers, but currently they are well suited for any reader (with a short attention span). They are like encapsulated "events" or plot within a chapter of reading ease and length, and are somewhat like "soap opera" style, so that one could put the book down easily enough and begin again for the next chapter. Like "chats" or epic poetry, it is a way to tell a story in a brief method. While it takes skill to do this, it is still less complicated than the novel.
The novel, then, is complex for the main reason: that it increases in complexity through various characters, plotlines with subplots, and structure may have a variety of goals and outcomes. It is much like trying to coordinate a massive wedding: you cannot forget which character sits where, and why they are treated as they are, nor can you forget the complexity while considering the value of each character's place in the plot, whether antogonistic, protagonistic, comically relieving, and so forth. All loose ends must be tied by finish. All ENDS must have some value to the plot. It is like trying to hold the tentacles of an octopus, with one hand!
For me, the novel is so very much as I have always found myself to be as far as expression. I have nearly all my life been an intense observer of people and their situations, their passions, and their behavior. Hence, I enjoy the complexity of life in all its "tentacles," though it is time-consuming (as far as research) and though it takes a lot of thought to place value on the many facets of life's movement.
Yes, this is only my opinion, but I think I've enough life experience and college education both, to have earned an educationed opinion here: I have been writing since I was 10 years old.
If it were not for the pompous attitudes of most "scholars," and "educators," I would have my Ph.D simply by my life's journey.