Why?

Sorry, I thought that would be obvious. First, the symlink works then regardless of the mount point. An absolute link may point to some completely different location if / is mounted to e. g. /mnt/install_target.

Second, the kernel needs to do much less work for resolving the symlink. See https://en.wikipedia.org/wiki/Symbolic_link#Storage_of_symbolic_links : «… the operating system still needs to parse the path name in the link, which always requires reading additional inodes and generally requires reading other, and potentially many, directories, processing both the list of files and the inodes of each of them until it finds a match with the link's path components. Only when a link points to a file in the same directory do "fast symlinks" provide significantly better performance than other symlinks.»

And third, both the ln command and the «ls -l» output is shorter. :)

The symlink "works", in that it's not broken but still doesn't actually work. Running these without using chroot or a container seems like a really bad idea.