I think the reason is that email is fundamentally asynchronous and unbounded — you can send anything to anyone, with no shared platform, no mutual agreement required. That's not a bug, it's the entire point. Every "better" tool that replaced it added friction by requiring both sides to opt in.
Interestingly, even AI hasn't disrupted it — if anything, AI has made email more valuable by making it easier to write well and filter noise. The protocol survives because no single company owns the incentive to kill it.
Email has morphed into something quite different from when it started, and was a good way for humans to send messages to each other. That use case has mostly been supplanted by SMS, whatsapp, the social media app of the day, etc.
These days, it's really only used for that purpose mainly inside organizations, and "public" email seems to be overwhelmingly marketing material and automated notifications, with only the sporadic message from a human being. It's kind of functioning as what people wanted RSS to be.
The cynic in me would say that it survives because it is an effective tool for marketing, and the subscriptions / recipients are controlled by the sender.
It’s simple, open, and relatively low-pressure compared to most modern communication tools. A lot of newer products try to improve it, but often they’re mostly adding layers rather than solving a fundamentally missing capability.
Also, it is your digital identity, there is no substitute.
Don't get me wrong, the entire thing is primitive and outdated AF, but as long as it works and can handle the traffic it is getting, it's not going anywhere.