Well I think I'm going to echo the thoughts of others here but let me see if I can expand a bit.
Yet nobody gets paid, nobody gets a job. Rather, we spend extra time lovingly making these creative brainstorms.
You almost answered the question yourself Arnav. There is a certain release in being creative. When I decide to do a campaign or a mission, its based on a need to have a creative outlet. I think that there is a story that I want to tell others and so I go and write about it for a while and I see if I like the idea or not. Sometimes, I like it just for the sheer fun factor of making a living, breathing (metaphorically) mission with events happening.
The biggest draw is to bring another person into your personal world and tell them what you think would happen if or what the next logical progression is.
It took me some time, but I eventually learned the ropes of mission and campaign design. Writing within an already established universe makes it more difficult because you want to introduce new elements without going over the old ones, or worse yet, contradicting what happened before.
With both Warzone and BlackWater Operations, I've gone over the story of FreeSpace 2 and sometimes even FreeSpace 1 and Silent Threat to ensure that my ideas fit the universe and that they are acceptable to it. This is perhaps a downfall with many missions I've played is that

of the things happening are not plausable, or at least not explained so that they are.
Shivans all of the sudden attacking has become cliche, although I did it with Warzone anyways but I think I presented my ideas fairly well. IF I do Shivans again, I want to do it differently from another angle and another perspective.
Another thing that has become cliche is the "Terran Rebels". We've seen

of missions where there are those gosh darn Terran rebels creating problems again.
No, you can't totally escape either cliche, but Ace showed in Twilight that an insurgency can be a little more than a bunch of bad guys with pesky ships that show up and ramble off annoying catch phrases.
There are always lots of issues and lots of critics on the ideas presented (I'm one of them, im not innocent of picking other peoples works to pieces either, but I do give mad props to people who did it right too) but ultimately when it comes down to making campaigns, its fun to tell a story and have a creative release.