the reason is government and taxes

in many parts of the world, telcos are basically state operated as frequency bands are allocated by them, and getting vendors to cooperate with the carriers in the circus of interoperability tests not only controls the devices allowed to be sold to be on their network, but provides revenue. when the margin for devices were high, the carriers make revenue by limiting their network to only allow the devices with the highest margins to recoup the costs of licensing and infrastructure cost.

carrier-locks are like a tariff for radio bands: Want your device to come on my network? go to this office and pay a fee, go to that lab and pay a fee before dialing in. the consumer isnt doing that so the vendors do it, thats what carrier locks are

As the price of devices go down, the margin to be made is smaller so the carrier locks appear to make zero sense.

source: was involved in selling a line of mobile routers around the world, its a tough business