In this context, why does he use "come"?
Oh! Quick. Here they come. Oxford English Daily Conversation Episode 2
Why doesn't he use "are coming" or "have been coming" ?
They are coming here / Here they are coming.
They have been coming here / Here they have been coming.
Update:
Can you please recommend an article or reference where we can use "Simple Present" with now events. So I can read and learn more about this usage.
Update 2:
I have read this article that describes When Should I Use The Present Simple Tense?
We can also use the present simple for short actions that are happening now. The actions are so short that they are finished almost as soon as you've said the sentence. This is often used with sports commentary, or in demonstrations.
He takes the ball, he runs down the wing, and he scores!
First I put some butter in the pan and turn on the cooker.
Is this apply on "Here they come"?
Best Answer
This is simply one of the uses of Present Simple:
"In exclamatory sentences beginning with here and there to express what is actually taking place in the present."
For other uses and as the source see here.