I don't think it's about resolution. I'm comparing a 15'' 1920x1080 laptop screen to my 2nd Gen Kindle. I think it's about brightness. Light-emitting screens make it harder on the eyes to read books. An iPad's screen feels similarly uncomfortable to me. The main benefit of e-ink is that it doesn't emit light and therefore feels almost like a paper book.

Agreed about the ergonomics: it's more comfortable to hold a kindle than a laptop. However, I'm not sure I could replicate the experience with a tablet: I'd have to lower brightness a lot, increase contrast, and shine a light on it, at which point LCDs stop being comfortable to read.