For me- and maybe I have ADD, or some similar disorder- hobbies only stay interesting as long as there's something to do. That's why I dabble in replicas and modding. The fun and excitement of tracking down rare, hard to find genuine parts, or building something to emulate/replicate/clone keeps me busy, and I'm constantly thinking of where/when/how I'm going to find that next odd, unusually rare part, or whatever. This keeps me busy, and keeps my interest piqued. When I had genuines, it was a case of- Go to the AD, haggle a bit about a measly 10% discount, buy it, wear it, forget about it and/or become bored with it. It was just never any fun for me.
I consider this a fun, creative little hobby- Lots to do, see, learn and experience. Lots of hands on interaction with modding, and lots of great people to share ideas, discuss and brainstorm with. For me... This is what makes this hobby the best.