This is quite rare where i live. I would imagine international airports fly abroad and want to make it clear, vs airports offering internal flights only. Might be wrong

Ask Grok

It's custom(s)

Counterpoint: With a gyrocopter everywhere is an airport but almost no where is international

Because they have customs officials available so they can be the first place an aircraft lands when coming from another country.

And why are there interstate highways in Hawaii and Alaska?