Because proper HTML is for describing the meaning, not what it looks like (that's what CSS is for).
If this was anything other than Microsoft, I'd say you have an XSLT file somewhere, which transforms that XML into HTML, and you would just need to locate that file and set the left and right margins to "auto" (and assign a fixed width) and the report would be centered.
(That box above was simply <div style="background-color:silver; width:350px; margin:auto;">I'm a centered box,<br />with left aligned text.</div>)
But - because it's Microsoft - they've probably invented their own way of doing it, just because that's what Microsoft does.
And it looks like they have, with their
RDL thing. So what you need to do is read their documentation on Layout / Page and see if you can set LeftMargin and RightMargin to auto, or if you need to calculate explicit values to use.
Didn't you mention you had a book on SSRS? This is probably mentioned in there somewhere.