Because children who spend their time looking at screens will probably find it harder to understand spatial dimensions. Perhaps they gain something else, like heightened social awareness, but not physically actively playing with 3D objects will surely take its toll.

But, the trend of increased screen time in my opinion is a bad one and would probably lead to lowered IQ. There's no reason to get stuck on a problem or spend time being bored. As children playing with Lego, Mechano, K'nex, etc, we taught ourselves to solve problems with no shortcuts. We were bored, we found a problem, we got stuck, we troubleshooted and we overcame. It seems like the younger generations of today are losing this Scientific process.

Probably the most concerning issue is that the reward system doesn't require pushing your mental abilities. You can see interesting output without having to put the work in. Because many of the websites/apps measure success in engagement, they literally want you to do as little work as possible to see as much interesting content as possible. Contrast that to pre-screen time, where if you wanted to play with the Lego model on the box you had to build it. Mechano would even actively put mistakes into the instructions to allow kids to problem solve.