They want to build something. The box is right there. But cardboard is the one material a child can't get through on their own: kid scissors stall on corrugated board, and the box cutter that does work is the one you'd never hand over.
So you stop what you're doing. You cut it wrong, because you're rushing. You tape it, and the tape peels off the dusty cardboard within the hour. The rocket folds in half. Someone cries. There's a pile of half-built cardboard in the hallway that you're now the villain for throwing out.
The alternatives don't help much either. Craft kits are one project, pre-cut, finished in forty minutes. Building sets have a fixed number of pieces and cost real money to expand — and nothing you build with them is big enough to sit inside. Meanwhile the tablet is always there, always easier, and it wins by default every single time.