I think you've hit the nail on the head here. An e-ink tablet needs the same SoC, battery, metal case, and software engineering that an LCD tablet needs. So if an e-ink display cost the same as an LCD, the tablet would cost the same. But because of the limitations of the display technology, the device would do less than the equivalent tablet. You can't play games on it. You can't watch Netflix on it. Browsing the web would be a chore that nobody would do except in an emergency.

For that reason, the devices don't exist.