Because HTML isn't XML.
HTML is based on SGML. SGML is an old standard, and is used to define markup languages for all sorts of purposes. XML is also based on SGML, but is completely seperate from HTML. HTML allows for a much wider variety of SGML features than XML does. XML's main features are a very simple syntax and draconian error handling, which makes it suitable for marking up error-intolerant documents and also makes it very easy to learn.
During the XML craze of late 20th and early 21st century, someone has the idea of making an XML-version of HTML; XHTML. The idea was that, since XML was already being used a lot in various applications (such as vector graphics (SVG) and mathematical markup (MathML)), this would make it possible to easily extend the HTML page to natively include such additional elements.
The only problem was browser vendors and page authors. Page authors were told that XHTML was the new black, and that everyone had to start using it, or they'd miss the cultural revolution of Web 2.0. Nobody really knew why they were using XHTML rather than HTML, except everyone else was doing it. As a result, everyone did it wrong (and still do). At the same time, the browser vendors couldn't exactly built their web browsers so that they were incapable of displaying XHTML pages, as long as their competitors didn't do it as well.
The XML declaration is not necessary, unless you use XHTML 1.1 (which you shouldn't). With HTML 5 supporting the general concepts that originally gave birth to XHTML, XHTML will die a slow and painful death over the next 20 years. Until then, HTML 4.01 is still the latest standard advisable to public websites.