I work for a telecom.
99% of the time this was because the cost of the phone is built into your plan. There was a serious risk (and still is) of fraud whereby the phone is fraudulently ordered to an address, the phone physically swiped, the customer never pays, and the telecom can’t recover the phone or its costs. More basically, it used to be pretty hard to get money from customers who just stopped paying. You could get a €2000 euro phone for €500, pay that up front, and walk to the local guy with a serial cable who unlocked your phone for €20.
Theres a lot more protections, technological and legal, that have slowed this now, but the profit is still high enough that jumping through hoops like embedding an ally in the contact centre or intercepting couriers is still worth it. Most of our phones are no longer locked to carrier as we just have better ways of dealing with it now, and all we were doing was feeding 20 euro to the guy who also sells vapes and buys gold.
I’m sorry the carriers you deal with are so shit but until mobile data transfer becomes a government utility (and let me tell you, there’s a reason telecoms are scrambling to diversify) they do have to make a profit. In most markets the margins are razor thin and new radio technologies (4G, 5G, 6G) are costing more and returning less.
So when poorly regulated markets let them merge into monopolies, or they cut costs by reducing human customer services, “based, I stole a phone from a shitty company” should hopefully be also followed up by you supporting legislation to make mobile data a government utility.
For reference I work in an EU telecom and our industry is heavily regulated. If software companies or supermarkets were hammered for what they do with the data we “just” transfer, they’d be a lot cleaner too.