As someone who is super nearsighted, the smaller screen on a phone is great for reading, especially in contexts like bedtime reading where I want to have my glasses off.

I have read many hundreds of books this way.

The problem with a tablet is that most tablets, especially the sort that are good for seeing entire as-printed pages at once, are too big for me to keep the entire screen in focus without wearing glasses. (with that said, foldables improve things here, since the aspect ratio bottleneck is typically width so being able to double the width on the fly makes such things more readable.