The obvious answer is that novels and short stories aren't interchangeable.
Most of the authors I know, myself including, have misjudged the "weight" of a story idea from time to time. Occasionally, I'll start work on a novel only to have it fizzle in a chapter or three when I realized I've already said everything there was to say, and in far too many words. That central-core idea simply wouldn't support chapter after chapter, no matter how many secondary characters, narrative descriptions, or turns-of-fortune I stuck in. Likewise, I've found myself in the middle of what I believed to be a short story, when it felt like someone exploded the walls of my house and I'm floating in the middle of a galaxy -- the world got suddenly much, much bigger.
Way back in my newbie days, a wise friend likened the process to finding the right type of wood for a sculpture. Balsa is soft and light, great for toy airplanes (like a short-short). Then there's pine, a bit more sturdy, and so forth, moving through oak and maple to teak and mahogany and ebony. Those latter types are the ones you want for a novel -- dense, intricately patterned, durable.
A novel isn't a short story that goes on longer. It took me a long time to learn this, and in the end, I needed someone to explain it in words of one syllable. Even "episodic" novels have an underlying (or maybe it's over-arching) structure that distinguish them from a series of short stories thrown together like pearls on a string. Decades ago, I tried to write a novel that way (the pearl-stringing way), beginning with a short story (published) and then a sequel to that story (also published). I loved every new adventure that I put my characters through. The problem was just that: each was a new adventure. Having the same characters and a loosely-woven ultimate quest did not weave these stories into a whole. Perhaps a more skillful writer could have done it, but not me. The result lacked what you might call "shape." It was a series of humps, not a single mountain.
So why would I want to do that, anyway, besides the obvious dictum that career is spelled n-o-v-e-l? Once I had several true-novels (of publishable quality) under my belt, I came to appreciate what novels do so well: generosity. Generosity to the reader, but also to the writer. A novel is to a short story as a marriage is to a one-night stand, however glorious that might be. Novels offer us the space and time to savor, to return, to create interconnections, layer upon layer of them. The visit to Pemberley might be accomplished in a short story, but breakfast with the Bennett family, chatting about the Netherfield ball, listening to Mary's awful piano playing, and analyzing the letter from Mr. Collins...those are possible only in a novel.
The expansiveness of a novel is not a justification for an indigestible expository lump (writerspeak for "way too much information for its own sake presented all at once"). As in a short story, every part of a novel must do a job, but there is more time and space in which to work. If I've fallen in love with a character, a family, a world, I want to share my delight in their company. I want to explore Middle Earth a bit, sing with Tom Bombadil, walk the twilit paths of Lothlorien, and sit on a sunny bank in the Shire before I go storming off to Mt. Doom.
If I've done my work as a novelist, all those moments, all those nuances and subtle connections all come together in a seamless whole. The thrill of getting it right is one of the best there is.