MathML: horrible

XML is plain text so that it can be easily interchanged between computers, applications, etc. It is true that some people have created mini XML-based languages that they expect people to type. This works ok for them because their language and what it describes consists of a few named options with values. As you point out here, even for this application XML is not a great language. For something like math notation, the things it represents are complicated and its XML representation is complicated as a result. In short, if the thing being described is simple, typing XML is ok but not great. If the thing being described is complex, like math, typing it is ridiculous. Most uses of XML are of the latter kind and are completely behind the scenes.