This article gets so close but it feels like it goes right past the actual point.
Profit motivated tech companies are weaponising psychology to dominate your time. There's nothing stopping them from turning your reward system against yourself. Plus given this time is generally being spent on passive consumption, of course people are gonna end up dumber—brains need exercise just like the rest of your body.
Regulation is basically the only way to remedy this (beyond actually nationalising these organisations) and unfortunately it's gonna have to be American regulation for the most part, which I'm not gonna hold my breath for under the next administration