Simple, non-evil answer: because it would severely impede spam prevention. This extensive writeup has details: https://moderncrypto.org/mail-archive/messaging/2014/000780....

Long story short, to control spam, you basically have 2 options:

1. Control who can send messages 2. Inspect message content 3. Impose costs

Signal and WhatsApp chose (1). E-mail (and probably any federated protocol) requires (2) or (3), and there is no universally-adopted (3) for e-mail.