I think for me it's the wide variety available that require different living conditions to be kept properly. You see a set up someone else has and are like wow, I want to do something like that. Then you get into growing out fry, etc. My tanks right now are all very different from each other. I'm betting when I finally have a house instead of an apt, I'll have a fish room with many tanks.
Also, I think there is a sense of accomplishment when you have nice tanks and healthy fish. There is also the challenge of doing something different, growing new plants, spawning new species, etc.
Would be interesting to see if those who don't frequent fish forums have MTS like many of us who post on forums do...you may be getting a skewed sample only looking at people on forums because we are into the hobby enough to post about it and share ideas and knowledge.