That’s because “is” is the third person conjugation of Be, not the second. Of course it sounds weird.
“Thou are”, and the actually correct “thou art” both feel much more natural.
The conjugation of Be, “is” is not used in the second person (you, be it singular or plural) any more than “am” is (“am” is first person singular form.) regardless of the plurality (or lack thereof) in the subject.
The correct Be conjugations for second person subjects are “art” (2nd, singular, archaic) and “are”.
So is thou/you plural or singular? I’m very confused. I’m not a grammar person lol. This conversation began because someone said thou is singular.