Email is one of those cursed standards where the committee wasn't building a protocol from scratch, but rather trying to build a universal standard by gluing together all of the independently developed existing systems in some way that might allow them to interoperate. Verifying that a string a user has typed is a valid email address is close to impossible short of just throwing up your hands and allowing anything with a @ somewhere in it.
Email is one of the very few success cases of the xkcd Standards meme: https://xkcd.com/927/ - and it's due to practicality and ingenuity on the part of people who made very creative parsers and placed real-world understanding behind every word of the early RFCs.
Without a unified email standard, the world would look incredibly different today, especially as it bootstrapped open communication between different countries and institutions in developing every protocol since.
RFC 2822, for the curious :)