Quite simply, because Amazon doesn’t want it to.  For the first 3-4 years of the Kindle Publishing program, we could tell a MOBI file to open wherever we wanted it to.  The cover, the title page, the prologue or preface. However, in 2014, Amazon changed that, because so many self-pubs (using Word, that is) were having issues setting their SRL (“Start Reading Location”).  To address that, they now override any prior settings, and forcibly set the SRL to be the first full page of text after the TOC (“Table of Contents”).  

So, even if we set your SRL to be, say, the Cover or the Title page, in your MOBI, that will be overridden, when you upload your eBook at the KDP.  (For our “ClickByClick™ Guide to Uploading your MOBI file at Amazon,” please click here.)  

To address that, you’ll have to ask Amazon to adjust it, after you’ve uploaded the book at the KDP.  We’re sorry—we wish we could help you with this, but we can’t. It’s out of our hands.  

Update: January 19th, 2019:  As of our latest tests, no amount of tinkering, by us or by Amazon personnel, is able to override the SRL. Plan the interior of your eBook accordingly.