Because that’s the Kindle default.  More importantly, the only way we could override that, for the entirety of your book, is to forcibly set the alignment to left-aligned, sometimes referred to as “ragged right.”  If we do that, it’s possible that you could be subjected to a “KQN,” or Kindle Quality Notice.  

What's a KQN, or "Kindle Quality Notice?"

That’s when Amazon sends you a nasty-gram, informing you that there have been complaints and you need to fix your book. This can result in some highly unfortunate consequences; they can put a large yellow notice, on your Amazon Sales Page, that informs the prospective buyer that there are problems with your book, and they are “working with the publisher” to correct the issue.  Sometimes, they will remove the book from sale, if they feel it’s warranted.  (We had this occur to a client that insisted that she didn’t want a TOC!).

n.b.: Amazon’s Publisher’s Guidelines state that a forced alignment—justified, not-justified, ragged-right, etc.—is prohibited, because it overrides any possible user preferences.

So, before you ask us to override the default, and create ragged-right text for the body of your book, (the main narrative), think carefully about it.