Simple. Since the major carriers these days offer "interest free installments" on phones, with subsidies or billing credits to make the TCO cheaper... then when this happens, the carrier: 1) loses no interest payments, 2) may lose installment payments, but this is a credit risk anyway - onus is on them to assess risk to a point of acceptability to them, and 3) get to not pay out the subsidies or billing credit.
All this amounts to is an additional layer of securing a loan. And pre-emptively so. "You cannot unlock this device because others may abuse this."
Assess identity and credit risk better.
I have little sympathy for the carriers, after having to fight Verizon over the claim that I, living in Seattle, and being an AT&T customer for a decade with 4-6 lines and devices, somehow decided to go to El Paso and buy a phone at a Walmart on Verizon, run it up making international calls and let it go to collections.
After supplying their (onerous, tbh) info around identity, police report, current utility billing and such, VZW said "We are still satisfied that the debt is valid and belongs to you, after reviewing your documentation with documentation that was supplied when the account was opened". When I said "well, given that you have verified my identity, and given that you state that the documentation you have from the account says that it was I who opened the account, I'd like to see that documentation". Verizon: "We cannot provide that data to you as it may violate customer privacy". Schrodingers account holder. It's me when they want to collect money, it's not me when they're concerned about sharing "someone's" info or billing records...