Do you mean why are you in the body you are in? Or do you mean why are you this version of you?
I may not be understanding, but I tend to think that I am myself, this particular observer, because of nature and nurture.
My genetics and brain architecture can only create certain versions of an observer. The environment I grew up in and those experiences shaped which observer I would be.
But I can also imagine that even with the same genetics and brain architecture, that if I had been raised in a completely different environment, then the observer I became may be a completely different person.
However, if consciousness were simply a fundamental field that interacted with humans, you may be the only you that comes out of your biology when that field lights it up.
Just some thoughts, though I’d be intrigued to hear more if what you are referring to as an observer is something else.