For me:

Better aspect ratio for large amounts of text. I just can’t imagine reading an entire novel on my phone.

No distractions. No notifications, no other apps I could be using instead.

Related to the previous point - e-readers are for reading. I’m much more likely to read a lot if I can compartmentalise “other stuff” to my phone. Not to mention other features like the ability to look up word meanings and change text size - all stuff in service of e-readers being “for reading”.

Page-turn buttons instead of a touch screen (depends on the e-reader).