And this is widely considered to be a bug and a source of insecurities, not a feature. The few remaining people I know who use Thunderbird use it in text-only mode in order to control its HTML-rendering tendencies.
Most people don't want a full-fledged browser in their email; it opens up too much attack surface, opportunities for "read receipts" and tracking cookies, etc.
That said, I think email is a good and underutilized tool, and it's unfortunate that more MUAs don't support features like digest handling properly. I think there's a pendulum that's going to swing away from public services like Facebook and back to direct-communication services (Snapchat might already be arguably a part of that) and email is already well-positioned there.
But forklifting the web and HTML into email clients is probably never going to happen.