Hypothesis: we are primed to expect ads to pop up, or some other interruption, when we read on phones, and even when those intrusions are held at bay, we have retain the expectation that they will emerge.
The state of the internet. Sigh.
Likewise, I keep my phone on Do Not Disturb at all times, and have it configured to allow numbers in my contacts to ring through in case of emergencies. Everything else is silent.
These things have significantly decreased the activity in the part of my brain that expects distractions and adverse UI patterns at any moment.