Taking example of Signal, I need to save each and every image manually to be me available outside its sandbox. Why is it like that rather than making it easy for me to use things that have been explicitly sent to me?
I have used another messaging app called Twin Me which also does something similar.
I always used WhatsApp earlier that allowed me access to the media downloaded in chats and let me take local backups but unfortunately they now allow only Google backups!
Assuming you’re on Android, this is coming from the OS. It’s still possible to save data in an externally-accessible place, but the default is storing things privately for each app. It’s annoying for use cases like yours, but it’s arguably better for overall security.
I thought at least open source ones would do it otherwise considering the freedom they represent.
Looks like it has not reached that point yet.
It’s not a matter of freedom, it’s a question of desired functionality. If anything, keeping your data in a sandboxed area by default respects your freedom more (that is, freedom from other apps scraping your data).
If it’s on Github, you could file a feature request to have that default behavior changed.