2 Reasons for me:
1) last iPhone unlocked usually still sells for close to retail price of new iPhone so net out of pocket to upgrade is like $25 for the phone
2) if you don't upgrade you are giving up the $200-$300 subsidy provided by the carrier every 2 years to stay on contract
So I think it's pretty silly not to upgrade every 2 years unless the above doesn't apply to you.
1) last iPhone unlocked usually still sells for close to retail price of new iPhone so net out of pocket to upgrade is like $25 for the phone
2) if you don't upgrade you are giving up the $200-$300 subsidy provided by the carrier every 2 years to stay on contract
So I think it's pretty silly not to upgrade every 2 years unless the above doesn't apply to you.