Just Curious

I really wonder what it's like to love only one gaming system above all others. I see so many people talking about adapting various settings to their beloved RPG, and I just can't grok it. Why not just play the setting in the system it was designed? I'm not saying porting settings to other systems is bad, far from it. I'm actually jealous of people who can enjoy a single system so much that they'd take the time to tinker and adapt it to a setting. 

Just curious