Because it is a word in its own right, and a plural word at that.

Consider:

Kalniņa kungs, Jūs esat cūka!
vs
Kalniņa kungs, Jūs esi cūka!

The second would be extremely hard on (at least my) ear, because we just were talking in plural form and we slam to the teikuma priekšmets in plural a verb in singular. Doesn't, as a rule, work in Latvian, I think.

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Yes, but why is it always plural in its own right when we are addressing a single person?
'Juus esi cuuka' manupraat izklausaas burviigi :)

but okay...

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That would be like saying "They is". :)

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To me, it's kind of similar to English use of 'their', when you don't know if the person you're speaking about is male or female. 'Somebody left their bag here', for example. You're speaking about one person, but the possesive pronoun is in plural.

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