Simply because the products available after a year will routinely outshine what I already have. Here's my development path since the iPhone v1 came out: iPhone 1 (had it 1 year) -> iPhone 2 (had it less than a year) -> HTC Hero (had it for 4 months) -> N900 (had it for 3 months and counting).

I've recycled all of the aforementioned phones though. They are all in use by people I know, who paid decent money for them (except my dad, who got a gen 1 iPhone for free, but he gave me a house a few years ago, so I find it hard to milk him for cash).

I do admit there is a certain amount of gadget-love involved as well...