Because I don't need another device. My iPhone battery lasts all day (it's a 16 Pro Max). I do agree eInk is easier on the eyes.

I have thought about it. Whenever I think about something like an iPad, the rational part of my mind says "why not just get a reader instead?". Well, the iPad would do more. Turns out, I don't need (or even really want) either. Though, my wife has an iPad and it has manga and books on it, but I never feel the need to reach for it over my phone.