I mean, gotta click, hold, tap or use some other form of selection to go fullscreen in almost any app on almost any device. Been doing it so long thats its just second nature to me. If I need a picture or video larger I look for the button or gesture to make it happen. I will admit, it seems like I have had to explain how to open pictures fullscreen in the messages app an inordinate amount of times. That being said, if it opened fullscreen automatically, we would have a bunch of people asking how to get back to the message screen because there are no “buttons” showing.. gonna have to tap sometime.