I agree with you in concept, but I think in practice people struggle with the self discipline, and that's kind of the fundamental problem with apps (and particularly algorithm-based social media). I've set timers to limit my usage of certain apps, including my Lemmy app, to encourage mindful use, but I can understand why someone might want or need more of an enforced limitation.

You might not replace your current phone with a "dumb" device, but when it's time to get a new device eventually, you could ask yourself if less-smart device might meet both your functional and other needs.

Edit: I guess to me this is kind of like: why are people overweight? They can just not eat as much. And while that's technically true - and advice I follow - it's apparently not that easy for everyone. If it was, we wouldn't see problems as pervasively as we do.