There are technical reasons for this. For example m, OpenType only supports 64k glyphs within a font. Which is enough for the BMP but nothing else (and counting

There are technical reasons for this. For example m, OpenType only supports 64k glyphs within a font. Which is enough for the BMP but nothing else (and counting ligatures that are necessary for, e.g. Arabic, it might not even suffice for the BMP). Then there are practical considerations. While Latin, Greek and Cyrillic are similar enough to warrant the same styles (serif, sans-serif, script, italic, and various weights) not all of them make very much sense for, say, CJK or a variety of other scripts. So having different fonts for different scripts that are still designed to go together is actually not that bad a solution. It does mean that for good typography you need a matrix of fonts based on style and script. Word includes two fonts per style for this, to treat CJK differently, which might not be enough, depending on the numbers of different scripts involved in a single document. But a) several dozen scripts per document are somewhat rare apart from Wikipedia's language list per article and font demonstrations; and b) good typography needs effort, this won't change.