Why doesn't ln -s tell that it fails when creating a symlink to an existing symlinked directory?

Because in the second ln it doesn't fail it creates a

symlink_dir/dir_2 -> dir_2

symbolic link

Do a:

ls -l symlink_dir/dir_2

And you'll see a (probably broken) symlink there.

That's how ln is meant to work if the target is a directory (or a symlink to a directory).

A third ln could fail because there's already a dir_2 inside symlink_dir (aka dir_2).