Most don't, some do. I have a large collection of music, others have a large collection of something else, be it photographs, movies, etc.
Youre in no position to dictate how people should manage their files. Youre also in no position to make wild assumptions of what 'most' people have. Face it, hard drives are expanding out of necessity. Don't make it seem as if 200gb is plenty for years to come. I keep all of my stuff on my laptop because its conveniant. And no, i dont worry about it being lost as I have an exact clone of my secondary hard drive in my macbook which holds all of my data, my boot drive holds nothing except for applications and OS X). I've gone even further and spread out irreplaceable stuff, like personal photographs, and spread them out on optical media as well as on two hard drives. So even in worst case scenario, what I can't create again I have three copies of.
I'm completely floored by your comments.