If this is the case, then why hasnt the rest of the world seen a similarly startling increase in pedestrian fatalities?
The rise in smartphones was a global phenomenon. By your logic, the entire world should be seeing similar rises in pedestrian fatalities, but they arent.
On the other hand, the rapid increase in vehicle size/weight has been fairly localized to the US due to regulations that incentivized such..
Here is Ny times analysis of it... https://www.nytimes.com/2022/11/27/upshot/road-deaths-pedestrians-cyclists.html