I got a kindle last year. Since then I've been reading (mostly novels) for at least 45 mins every day. Some weekend days I read for like 2-3 hours. It's been wonderful. My iPad Pro has been collecting dust for months.
The whole time I'm reading something on a phone my brain is constantly pinging me about all the other things that I do on my phone: emails/WhatsApp/Google word I don't know.
So our conscious control centres in the PFC are always firing to mediate these little temptations. There are less of these temptations on a e-reader.
So much stuff can happen on a phone that the brain can't relax and focus on a single task.