It's not, and it has never been. The original unix time was "the time since 00:00:00, 1 January 1971, measured in sixtieths of a second", this got modified multiple times until it settled upon the number of UTC seconds since 1970-01-01T00:00:00Z, meaning it's non-monotonic, non-continuous, and not based on "physical" seconds.
It's not, and it has never been. The original unix time was "the time since 00:00:00, 1 January 1971, measured in sixtieths of a second", this got modified multiple times until it settled upon the number of UTC seconds since 1970-01-01T00:00:00Z, meaning it's non-monotonic, non-continuous, and not based on "physical" seconds.