When an explanation invokes God, what is the difference between that and “what’s just the way it is”?
Apples fall because space-time is warped in the presence of matter, and that’s just the way it is.
Apples fall because space-time is warped in the presence of matter, and God made it that way. Why did God make it that way? That’s just the way He made it—i.e., that’s just the way it is.
The advantage of Naturalism is that it leaves out the unnecessary steps in that description. You can add God, or the flying spaghetti monster, or Kant’s categories, or Schopenhauer’s will, or Hegel’s Idea, or whatever else you want to add, but it still only pushes the question of “why?” around, rather than answering it. Ultimately, the question of “why”, if it is not really a question of “how”, is a spurious question. It doesn’t have an answer, because it is not a legitimate question. You may as well ask why the apple exists at all, why does it exist as an apple, why are you able to perceive it, why, why why - to no end. There is no purpose to apples or appleness or falling apples or children who ask “why” (and children ask “why” in a way that does not always presuppose purpose).
Compare idealist versions of the thing-in-itself. They can never answer what the thing-in-itself is, because they have invented it in order to meet a demand of an idealist system, and not because they have any actual knowledge of the thing-in-itself - knowledge they deny is even possible. So why say that it exists? Why is it not enough that the apple is there in front of you, and had (or, for a pragmatist, is even constituted of) whatever properties you can perceive or infer about it and relations it has to other objects, but you must insist that there is some mysterious, nebulous “apple-in-itself” that is the cause of these perceptions and inferences and properties and relations? Why must there be a noumenon when the phenomenon works just fine on its own?
It seems like “purpose” just a left over from when human beings insisted that the world was filled with spirits and ghosts and daemons and gods. Now we know a little something about space-time and why an apple will move when it is near some massive object in space-time - the “why” you should be asking is “why do we need a gravity-spirit or gravity-daemon called ‘purpose’ to make gravity work, when it works just as well on its own?”