> The time used for "Unix Time" is actually UTC
Unix Time is not UTC; it is a linear representation that is surjective onto UTC. It is easy to map between the two but they are not the same. They are not even isomorphic or bijective because leap seconds have no unique representation in Unix Time.
The Unix Time representation gives up full equivalence with UTC in order to provide some useful guarantees (days are always exactly 86,400 seconds long, midnight always satisfies x % 86,400 == 0, etc), but also creates other surprises (time can go backwards when a leap second occurs).
The time used for "Unix Time" is actually UTC, which is a standard that is specified in international standards that predate Unix. It is specified in current international standards for everything from aviation to HTTP requests.
If Unix tried to use something else, you'd generate massively more confusion for every Unix developer as they were having to figure out the current conversion to what is actually required for interacting with the rest of the world.