It's the opposite - with a physical button, you just put your finger on it and keep it there while reading, so that you only need to push slightly to get to the next page. Whereas with touch screen you need to actively move your finger back and forth every time (since keeping it there would block the screen, and in any case you can't really rest it on a capacitive screen without it registering as a touch).

For one-handed reading especially, that alone makes physical buttons significantly more ergonomic, provided that they're placed such that your thumb rests on them naturally when holding the device.