I think a lot of the time the reason I may not go through with fleshing out a story to a random premise that came to mind is because I’m afraid of becoming unable to get back into the groove of writing what I’m currently writing as a main story of focus.
It can also be because although I have a idea for a story, I don’t have a vivid image of the main character or even side characters at that yet. It means I have to go and expend a ridiculous amount of energy to think up these characters, how they talk, how they interact with each other, their motivations, back story, etc. That being the case, I’d be concerned about other characters I currently have a clear image of in another story I’m writing being pushed out of my head. I’ll slowly forget character interactions, events, etc that happened in another story I might be writing at that time.
That’s why a lot of story ideas don’t get fully fleshed out for me I’d say. I just write the first chapter or two out in the event they may prove to be implementable/used in an unexpected way at a later date. Rarely do these 1-2 chapter stories ever see the light of day unless I put them into my dumping ground episodic story filled with all sorts of atrocities.