In professional works, it's usually because the story feels like it exists because an executive said, "make a story about [X]! It's big right now!", or "Make a sequel to [X]! People want one!" and some mediocre writer got out a beat sheet, filled in all the beats mechanically and created a roughly functional, but completely soulless execution of a story about that thing. Everyone's a stock archetype going through the least imaginative set of derivative plot beats to learn a lesson the writer doesn't believe in, but thinks people will react to, and then throw in some emotional music or imagery and ship it off to print.
In amateur, small press and self-published works, it's usually because somebody came up with some OCs they think are so very precious, and thinks that a good story is a collection of basically any old stuff happening to their OCs, even if it's completely aimless and pointless. They think that if the superficial details are a combination of things that they haven't seen put together before, that makes it a good original story, like "Aha! You've all seen stories where a handsome man is a superhero! But what about... a story where a purple dog is a superhero!?" It's like... yes, that is a combination of things I haven't seen before, but if it's not using being about a purple dog to say anything new and interesting about... I don't know, anything! The superhero genre itself! What it means to be "human"! How people react to strange things they don't understand! and instead, it's just a generic superhero story... what's even the point?
In the first case, it's not a lack of skill, but just a laziness and lack of ambition or artistic expression. It's soulless "art as a product", and I've worked on stuff like that to pay the bills and now know the mechanics so well, it's hard to enjoy any more like I might have done as a teenager.
In the second, it's a lack of creative development. It's like being told a story by a child; you can be like "awww, that's cute! What a creative idea!" and want to help them grow, but you wouldn't choose it if you wanted a really good tale to get lost in and feel inspired by, because it's too random and without a unifying point to make.