Recently I have seen a lot of frozen caliper slides while replacing brakes. Some of these calipers were relatively new.
Usually I can free up rusty frozen calipers with an ATF/acetone mixture, wire brush the pins, clean and lubricate the slides and reassemble the caliper. Sometimes, the slides are so rusty I can’t free them up without breaking them.
New calipers usually come with a light grease on the slide pins. I have found this grease is insufficient to keep the slides from seizing, especially on cars exposed to road salt.
When I do a brake job I always remove and lubricate the caliper slide pins with caliper grease, even with new calipers. I also lube the contact points of the pads with caliper grease to prevent them from being seized in the calipers. Sometimes I have to dress the tabs on the brake pads with a file to open the gap enough to provide a slide fit in the caliper.
It is also important to use never-seize on all the hardware when reassembling the brakes.