It is private to the app. Mapillary does not want you to have access to your own photos. And the security model of Android explicitly considers the phone owner and user as a threat and therefore gives the apps the opportunity to hide data from them.
Only if you have rooted your phone, you will be able to access that storage.
I read somewhere in this forum that the mapillary app might re-enable accessnat some pont, but since about 2020 there has been no change.
This request makes sense to me - and we will add this to the backlog (making it possible for you to see the Mapillary images folder even for non-rooted devices). Thank you for the suggestion!
Mapillary on Android now supports saving images to shared storage, thanks in part to this feature request! Ensure that the “save images to shared storage” option is on in settings.