I am just trying to wrap my head around the reason people want an unlocked phone.
I hear you. It's not a big concern for me either as I tend to stay on one carrier. I've been with Sprint for 15 years. Planning on leaving, but even if Sprint did unlock my phones it wouldn't do me any good on T-Mobile.
I think the thought process is that certainty of being able to leave without an issue if your service is not to your liking. It's the same thought process that those (such as myself) have with unlimited data. I never worry, never obsess about any of my data usage because I don't have to. I'm free to use as much or as little as I wish. Likewise, being free to leave your carrier without having to jump through a bunch of red tape and hassle is a security that a lot of people want.
Additionally, people also think "It's my phone, I should be able to just take it to any carrier I wish. The carrier should not be able to prevent me from using my device on another carrier."
Now, I'm not providing any defense here one way or the other. I'm just stating what's been my observations since I joined the forum here as well as what I've seen on other forums.
Sprint's primary reason for locking phones down is that they have a $30 billion agreement with Apple. They have to sell that amount worth of iPhones. So, they lock them down because they lose money if people take their service to another carrier. Making it extremely difficult to do that prevents the average user from leaving. Sprint's pretty used to ignoring pissed off customers because that's usually all those customers can do to Sprint is be angry. Sprint get's it's money either by ETF or full price payment.
P.S. Your frustration about unlocked phones is the same I felt one day when no less than four threads about unlocking Sprint iPhones showed up in close proximity to each other. That's one of the reasons I made my own thread about it and put the link to it in my signature. No one wants to accept "no" when it comes to trying to get their Sprint phone unlocked for some reason.