Because they're is a contraction for they are.
:)

English Teacher
Why?
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.
-ESL Tutor
InspiredByEnglish
Their not coming is no skin off my nose! I didn't want them here anyway!
(Context makes so many things possible!)
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
They're not coming. Hoho!!
They're is the right answer.