I think it’s a class divided too- (financially) poor parents give their kids their phone but richer/more educated parents don’t.
It could also be that the parent wants to be on their screen at the same time, or wants to be on Instagram later into the night. There will be some correlation with work, but I doubt that explains most of it.
Almost every other parent I speak to are well aware of how detrimental screen time is to their kids, and yet often still use devices when they're too tired for much else.