Because you're wrong. Your numbers are a complete falsehood. I would know.
Full Retail 256 GB 7 Plus = $969+tax.
AT&T Installment Plan = $48/mon.
After 12 months I have paid $576 and trade in the phone hassle free for a new model.
I've also saved an additional $300 on service over the 12 months ($40/mon vs. $15/mon device fee) compared to cost of using the exact same phone on the same AT&T plan.
If you were to attempt to sell your full retail unlocked phone in order to upgrade at the 12 month mark, you wouldn't even come close to recouping the costs that I have compared to your uninformed decision.
If you really need an unlocked phone for switching carriers, as some people do, then its a perfectly legitimate decision. But don't pretend like it is cheaper than the leasing options because its not even close.
So let's get this straight....
You pay $48/month just to finance the phone on top of whatever your service plan is, so I'll assume low and say $45/month.
I pay $869 for a 128GB model and $45/month for service.
At the end of one year you've paid $1116. I've paid $1409. Advantage: you.
At the end of two years you've paid $1116. I've paid $540. Advantage me.
At the end of theee years you've paid $1116. I've paid $540. Advantage me.
Let's total it up...you pay $3348 over three years. I pay $2489.
I mean, if you want to pay out your behind for the latest and greatest every year go ahead, but my brain tells me there aren't enough features to upgrade every year and it's money down the drain.