Because it's what I longed for when I was growing up. I felt lonely, and knew that I didn't want any of my DCs to feel that way.
I had friends who had big families, and their houses always seemed to be full of fun, and were never quiet and boring like mine was.
I do have a sister and we're extremely close now, but growing up she would rather that I hadn't been born, and I always felt a bit cheated. There was an almost 6 year age gap between us and she never wanted to play with me.
IMO, this also explains why I wanted mine close together, although this didn't work out in the end, and there is a big gap between DCs 1 &2. It's worked out well in the long run, but the older my DC1 got, the more I stressed that she would be an only, and that if she wasn't there would be too big an age gap.
How many DCs do you have, OP? How many would you like ideally?