It's a learned behaviour.
My lad is 3 years old and we've never put a phone on in front of him to watch anything. We'll put something on the TV for him when we're with him, and we're talking through it.
We'll be in a resturant and a kid the same age as my boy will be just sat their watching some shite on the tablet while the parents are either on their phone or doing something else entirely. No thought or process to what the child is doing or any engagement.
My boy will kick off if he's bored. But there are plenty of things around. Give him a toy car, sheet of paper, or even walk around and it occupies them. Or you could put a digital nanny in front of them and be done with it.
Parenting is fucking hard, especially as we're in a world now both parents have to work to stand still. If you've had a hard week, and a child who's particularly crap today, I don't blame them for opting for the digital nanny.