Why Is a Non-Connected Interface Up?

The loopback internal command may be configured on the interface to enable internal loopback detection. Therefore, the interface is Up.
You can run the display interface gigabitethernet 1/0/1 command in any view to check whether internal loopback detection is enabled on GE1/0/1.
display interface gigabitethernet 1/0/1
...
Port Mode: FORCE COPPER
Speed : 1000, Loopback: INTERNAL
Duplex: FULL, Negotiation: ENABLE
Mdi : AUTO, Flow-control: DISABLE
...
The command output shows that internal loopback detection is enabled on the interface.

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