Well, lots of reasons. I like trying out different knives as tools to see what works best, a knife can do a lot, it can create a lot and I would like to... know about them!
There is another reason - as anyone with an addictive personality knows. I wanted to go a different direction in my life, and knives did help me do that and provide a concentration point years ago. They hold a sentimental association of the change I have been cultivating the last 5 years and the man I want to be (Prepared, helpful, ready, having a solution).
I did purchase them somewhat addictively but have tempered it and made rules for myself, set amounts to keep me honest. (Hint - Victorinox is a MUCH better knife to satisfy that addict side as opposed to a 200+ Spydie)
Like the grocery store or the casino, knife collecting promises good times, but without restraint, I will get ruined.
I just enjoy tools but knives are a step beyond with steel, design, materials... I want a SAK with my special scales and a special Vic and Spyderco blade and if I dare imagine it, a Microtech UTX-70 inside it where two layers would be. The idea of an auto SAK, locking or not, is my dream.