I’m working with Word documents in different languages, and few people take the care to properly tag every piece of text with the correct language. What you’re proposing wouldn’t work very well in practice.

The other historical background is that when Unicode was designed, many national character sets and encodings existed, and Unicode’s purpose was to serve as a common superset of those, as otherwise you’d need markers when switching between encodings. So the existing encodings needed to be easily convertible to Unicode (and back), without markers, for Unicode to have any chance of being adopted. This was the value proposition of Unicode, to get rid of the case distinctions between national character sets as much as possible. As a sibling comment notes, originally there were also optional language markers, which however nobody used.