So... An eInk laptop wouldn't be for media. It'd be a different product with different pros/cons. Easier on the eyes. Better in sunlight. Easier on battery. Quite usable for reading, emails, code and any "mostly text" use.

To the OP's original point "why doesn't this just exist..." IMO, it could, but it would need to be a different product class. Just sticking the monitor onto a windows machine isn't good enough. It needs its apps/etc. The market is probably much smaller than the phone/tablet and laptop/desktop markets. Perhaps potential vendors don't think it's big enough to support the necessary OS, apps & such.