How's that? Because it's going to be basically impossible to single out this variable in a kid. First, not all 'screen time' is the same.

You see the traits in a kid you know has unlimited screen time and ascribe that to the screen time and not the other parenting decisions that lead to those traits. For example, screen time as a babysitter, only TikTok or mobile games, never talking about their screen time, etc.

There's likely kids you don't think have unlimited screen time that actually do, but because of other parenting decisions, it doesn't result in the same traits. For example, encouraging their kid to read with their screen time (books are on kids tablets now), watch science videos, talking to their kids about what they're doing with their screen time, doing screen time with them, etc.

This is why psychology and sociology studies are so hard, because it is almost impossible to isolate a changing variable in testing.