why we omit "to be" in subject + to be + verb + ing when writing photo caption?

Question

Hi, when writing photo captions, I often see people are writing sentences like this:
"My dog and I enjoying the sunset at the beach" instead of "My dog and I are enjoying sunset at the beach".

Why is "are" in the above sentence omitted?

Answer 1

Because it is a caption to the photo, not a sentence.

Answer 2

The caption is a description, not an exposition.

Answer 3

Because a photo caption is not the place to write a novel, but rather a succinct description of the context of the picture.