Not everyone has an email.
A businesswoman once proudly shown me a dumbphone and said that she does not have a personal email, only a corporate one. That persuaded me for a while, until I learned that her husband was prosecuted for money laundering around the same time. So yes, not everyone has email, but those who don't are few and they have very good reasons.
Email is insecure
Aside from spam and automated emails, pretty much all email typed interactively in an email client is encrypted between the sender and the receiver.
We could have a long technical discussion here about the opportunistic encryption of STARTTLS or about the market share of Google, Microsoft and Apple, but the reality is that email is protected for all practical purposes that matter to ordinary people. That is, it is impossible to view and modify personal emails in transit.