I'd say that I got into designing things for FS the way most people probably do. We want to fly enough to buy a simulator (even if we fly IRL), we buy the sim and start flying. Then we get tired of default airports and buy a few, perhaps, or we get tired of the default aircraft and feel the same desire to buy addon planes. After a while we settle into an idea of what else is missing that isn't available - developers neglect a region of the world for airports, or don't make a payware Grumman AA5/Cherokee 140/Robin DR400 etc etc that we learned to fly in. We get frustrated especially when we realise that the addon will never be made unless...
And the rest is history.
We either develop a taste for making addons, or our desperate need to see variety in the sim keeps us developing even if we would rather be flying. I'm in the latter category. I make AI planes because they're not available and I want them
now! 
If someone came along tomorrow and released a better one, I'd use theirs in a flash. Until then, I have different shapes in my sky that do the job for me even if they're not perfect.
A friend of mine from years back wanted semi-random flight plans and developed a script that swapped flightplans week-by-week so that an AI An-225 wasn't at Zurich every Thursday. I think he hoped I'd code the flightplan generator for him, because creating and validating all that data is a pain. So I guess we all have different frustrations about what we see the sim as lacking, and some of us get so gee'd up about it that we do something about it in our own professional or amateur ways.
Somewhere in that process, a few people will decide that development is actually more interesting, so flightsim really just bootstrapped an interest in, say, 3D modelling.
I actually want to fly more than develop, so my present process involves flying my AI Falcon 2000 between London and Paris (various airports) while I'm doing Gmax things. That way, when my head begins to ache at Gmax's obtuseness, I can decouple the AP and commence my descent into Le Bourget and get some downtime.
I should also say that I have a list of GA planes I intend to make. It'll be interesting to see how I feel when I reach the end of that list.