Kids have much smaller fingers, and so can use the touchscreen as a higher-precision input method than touchscreen. For those with adult-sized fingers, the precision sucks.

There's still the feedback issue though. How can anyone tolerate the poor control feedback of a touchscreen pretending to be a real controller?

The lack of buttons for additional fingers also seems like an issue. Unless you use the awkward claw grip that I know some people did for certain PSP games, you're limited to just your thumbs for input.

There's actually more controllers now than ever before that have even more buttons accessible to non-thumb fingers. A phone just seems like a poor choice for a game like Fortnite.

My kid doesn't mind, he says he doesn't need feedback.

On the other hand, mobile screens have multitouch and drag/swipe actions, and that's a real complaint when he's using a controller with physical buttons.

Eh. Kids have dexterity issues, depending on where we define the kid age range.