You are here. The reason for which may seem a mystery, but the most enlightening question on the matter is: would you rather be anywhere else?
The closer you get to comprehending the Ultimate Reality, the more you will appreciate your current state! To get the point across quickly (though this is not precisely whole and true), answer the why not with love, or play, or growth but with fear. "Fear" of the state of being everything, everywhere all at once. (Fear is quoted because it is closer to dislike or desire-not-to-be-like-that. It's hard to describe Being the Ultimate Reality as "bad" or "scary" since those words start to lose meaning, but you can get the feeling that One might prefer not to Experience that, even if you cannot put your finger on why exactly)
And so, One convinces Oneself that One did not exist at a time (by creating time itself) in the past, and also will not exist at a time in the future. What a blessing to be temporary! What a blessing to forget! And so One Believes in Birth and Death with some time in between (80 years, 800 years, 800 million, they all are arbitrary compared to infinity). So One begins existing at a time, but if One is still everything, then what acted upon One to bring about its existence? The external-internal divide is created to satisfy this.
Now there is a Me, and what does this Me have? Well a Body, of course to create separation (again arbitrary). Picture a physical (4 dimensions created earlier) reality that contains a lone Me. "Who created this unique Me?" asks Me. "The entirety of the external is so different ". Let's add other Me's that also Experience (or at least appear to Experience), and for good measure let's give them similar Bodies. And further let's single out some Me's (two, again arbitrary) that take responsibility for creating Me. And now Me is further insignificant, apparently only one part of one layer of the fractal process of Me's creating Me's, a process which seems indifferent to any "individual" Me.
Me, being astute, wonders where Me's in general come from, and so we get Me's with a huge diversity of Bodies (animals, plants, rocks etc.) and an apparent diversity in ability to Experience. In addition, the more similar a Me's body is to Me, the more likely it is to seem to Experience like Me (humans -> ~apex mammals and some birds -> rest of mammals and birds -> reptiles & amphibians -> plants -> rocks etc.).
This is a wonderful trick to sell the idea that Experience comes from the "physical world" when truly it's the other way around. The framework shifts from "entity" ( for lack of a better term ), to "physical entity". Both of which Me is just one such example of. But with the latter, there comes the question of what are physical entities made of. (It is possible to be made of something, rather than just Be. In fact, fully buying into this illusion entails Believing that it is not just possible, but even fundamental to know what an entity is made of in order to know what it is.) In this way, most "external" Me's stop being seen as Me's, they are what you might call physical things or physical entities. I doubt you consider a rock to be as much of an entity as yourself, but note the story in the above paragraphs (re-read it to double check): the external-internal divide turned everything into Me's and picked out a singular for You, never was anything added after that was not a Me! No physical reality was described, even though we had birth, death, parents, a whole animal kingdom, and a habitat for them to dwell in; those were still formless entities, closer to Ideas or Concepts or whatever you would call the Ultimate Reality that we started out with.
A clarifying thought exercise: imagine starting with a similar body but it has 4 legs instead of two. To remain convincing, that is what everyone else looks like as well. Also, most animals also have a 6 limbed skeleton. Imagine evolution as a process to believably explain why you exist the way you do, instead of a process that is the reason you exist the way you do. Essentially take our normative understanding of causality (that things create phenomenon) and flip it around (phenomenon necessitate things). Maybe the planet you live on and the laws of physics are also a bit different. Maybe earth is a cube instead of a sphere. Of course the specifics here are hypothetical, there is more than one way to skin a cat (~infinite here).
You can also go back and tweak the other arbitrary choices. Try to imagine space as 2 dimensions or 4, and every aspect that could change. Imagine if you live for 100 million years (note that years means nothing technically here, but understand the spirit of the question), maybe planets exist a million times longer than they do (that would make that reality decently similar to what you experience now) but another way to skin the cat is that planets exist for the same amount of time as they do here, and you are now more like an elf from LOTR (of course this necessitates physics changes, but that is no bother). Or imagine that you are the only human on earth. You would probably need some catastrophic event (nuclear war, etc) to have happened right before you were born, and your mother to have died in childhood, so that you the reason you're the only human on earth is that you are the last human on earth.
This highlights exactly why I told the story in the way that I did. There are some things that are much less arbitrary. Being the only human on earth as the first one, spawned out of nowhere, doesn't work. As in, it's not believable! It's not stable. You have a tendency to go back to the Ultimate Reality. What happens during a strange dream where you realize that things don't add up? You wake up ;)
So why, then, do you get these particular set of arbitrary choices? They seem so specific, especially when you fully consider all the possibilities. Could you change them (change "physical reality" to be whatever)? I'd say yes, if you truly desired, it would be already done. A word of caution: to be small is a blessing, to be insignificant is a blessing, to struggle is a blessing, to be ignorant is a blessing, to fail is a blessing. There is so much to Appreciate, and Where/When/How You Are is exactly Where/When/How You Want (and have Chosen) to Be. Could even the Ultimate Reality Know Why it Is?