For people interested in this topic, it probably goes back to a setting called UMASK on linux, which specifies default permissions for new files. The default value of "022" means "take away no permissions from the owner, and take away write permissions from the group and the rest of the world" which means that file owners have full access, everyone else has read access.
They say it's to make it easier to share files with other users. I say it's obvious that this logic predates times when we had web browsers and other tools capable of saving passwords to disk.