Remember that "이것" technically means "this one," but is just being used as this in "this" phrase and "은" is the subject marker. "거기" means "that (which is next to the listener, and not the speaker)" and "에" is basically the location marker.

So this phrase depends on the context of the sentence and what question is being asked. A situation where this would make sense would be if someone asked:

Q: 그것은 어디에 있습니다? A: 이것은 거기에 있습니다. Q: Where is that (the thing/item next to you)? A: This (the thing/item next to you) is there (the thing/item is from/located next to the person asking the question.)

Hope this help!

Both 거가 and 저기 mean "there". I found the following explanation on an online forum;

여기 = here = next to me (to the person who speaks) 거기 = there = next to you (to the addressee) 저기 = over there = far from both of us.

So 저기 is not used because it refers to something across the room...while 거기 may be within an arm's reach.

Maybe because 저쪽에 indicates a direction and 저기에 does not (rather location of something). 이것은 저쪽에 있습니다 = this is that way (direction) 이것은 저기에 있습니다 = this is there (location)