Unfortunately, most tablets (and particularly not the Amazon Fire Tablets) do not have infrared “proximity detectors” that many smartphones have.
Smartphones have this feature in order to to turn off the screen when held up to ear, or to turn on the screen when you wave over them… Though proximity sensing “broke” on my Moto X PE several weeks ago with no obvious explanation. Perhaps the infrared LED or sensor broke.
Regardless - Maybe if tablet makers knew what we were using them for, they would add such a feature. But I think even the Amazon Show doesn’t do this… Probably because it is meant to be voice activated.