The devs reply about this multiple times in every thread.
It won’t be added because Obsidian is a plain text editor. It works on plain text files not encrypted files.
There are many good benefits to this - other applications (including apps on mobile) can interact with the same files. I use Automate on Android to append to log notes multiple times per day, and I couldn’t do that if the notes were encrypted at rest.
There are already plenty of existing methods to encrypt your files which you can use alongside Obsidian to encrypt your vault - Cryptomator is a great one which has been mentioned in this thread.
Alternatively there are other note taking applications which do encrypt everything at rest - StandardNotes and Notesnook would be two good examples.
For further reference:
And: