Not just smartphones, but the internet in general in my experience.
I'm not much of a smartphone user, I have the latest phone but I'm not a power-user at all. But I have the same issues talked about.
For me it's related to how I consume information, it's very non-linear. Wikipedia is probably the best example, you read a page, then come across affiliated topics and click those, you go down the information tree on that topic, then come back up to finish the original one. Such that reading about the history of say a particular city, you'll end up reading about other cities, about pagan religions, about warlords and their particular tactics, about irrigation systems, political theory etc etc. It's extremely non-linear.
And when I research a topic, I find I consume information similarly. When I think about say the environment, I tend to have many tiny questions popping up, and get them answered. This is akin to having a private teacher who has answers to all of a kids' questions, versus a school teacher who has got 60 minutes to cover standardised material in a linear fashion.
Books are extremely linear versus say wikipedia. You flip pages one by one, there's no skipping or switching subjects built in to the design, like say wikipedia or the internet more generally as a source of information. And my brain just can't handle having questions pop up, and wondering about things, and just having to ignore my curiosity to get the book finished. It takes away from the enjoyment for me. Even when I read fiction, and there's a character walking in Paris and looking at the 'famed xyz', with a brief description, I want to look at it! On the internet I'd immediately google it, in a book I just have to accept that I've got no clue what it looks and shut down my curiosity. At least if it exists in the real world, I have no issue with letting my imagination run wild in fiction.