Because they're is a contraction for they are.
That's what I thought. ;-)
Right! Some of my students have a hard time with this. Since "they're" is a contraction, you can use either "they're" or "they are" in the sentence. If putting "they are" in the sentence does not make sense, you are probably using the wrong "they're/there/their" term. :-)
Their not coming is no skin off my nose! I didn't want them here anyway!
(Context makes so many things possible!)
They're not coming. Hoho!!