To put it in a nutshell, because I enjoy it. But that doesn't quite say it all. I think it has to do with being able to see the beauty in the machine, the effort someone took to make everything work "just right". The curve of the blade, the sweep of the guard, the way the whole thing fits together to form something else, better than what you started out with.
I Started out like most, with Boy Scout knives, a pocketknife when I was 8 and a sheath knife later. Both of which I still have, a little worse for wear, but still around. I learned to appreciate what a knife was and could do. And more importantly, what NOT to do with it. Over the years I accumulated more knives, kept some, destroyed some, and got rid of others. It wasn't until I joined the service in the late 70's that I started to really look for serious quality in a knife and as luck would have it, I managed to find several knives that had those qualities I was looking for. None of them were anything special; at least I didn't think so at the time. They were just good sturdy knives that held an edge and, more importantly at the time, didn't cost a fortune.
As I gained in experience, and pay, I started looking at more expensive, better quality knives. Some were worth the money, others weren't. The custom knife market had taken off by then and I looked with envy at the pictures in the magazines and bought "semi-custom" knives from A.G.Russell and the like. After the military I was more concerned with feeding my growing family than collecting knives, so my small collection went into a box in the closet that I dragged out every once in a while to clean and dream over.
Over the last 10 years I've gradually been able to acquire more knives but I've tried to be a little pickier. I still buy knives I like and want to use, but I've also tried to make sure they were a little more unusual, the first production run, or the last one. A limited production or a special finish or scales or even sheath. So far, so good, but it wasn't enough. Within the past year I have finally been able to enter the custom knife arena in a small way. I went from nothing, to a half dozen in about six months. I haven't neglected the production knives, but I'm even pickier now than ever before. I doubt that I'll ever have a major collection of custom knives, but wherever I end up, the trip has been worth it, and fun to boot.
