[social] "the" before US and UK

[Kazunari Hirano]

Hi all,
:)
I live in Japan.
We never say "I live in the Japan."
But I sometimes or often see that people say or write:
"I live in the UK." or "I live in UK."
"I live in the US." or "I live in US."
Why sometimes or often can US and UK have "the" before them?
:)
Thanks,
khirano