Because in order to maintain backwards compatibility from 1971ish onwards, each new standard has been awkwardly shoehorned onto the previous standards. And even

Because in order to maintain backwards compatibility from 1971ish onwards, each new standard has been awkwardly shoehorned onto the previous standards. And even then lots of big email senders don't properly follow the standards, so displaying email correctly requires being able to discover and implement thousands of unwritten and undocumented rules.