Because schools are fucking stupid is why.
All the research suggest that screens are not overall a good way for kids to learn content. There are limited areas where they can be useful, particularly showing some good quality film content in very small amounts.
But if there was a choice between no screens, and a lot, in a school, the kids would be better off, by far, with none.
I was at a school recently to do a puppet show, while the kids were eating lunch the teachers in every bloody classroom had movies on the screens, or videos of Disney songs with cartoons. It's because they can't manage the classes.
And it's easier to put them on some maths computer game then actually teach them maths. It's not effective, but the kids have some a shit attention spans so teaching them is a losing proposition, plus they have no real options for disapline.