Because it has nothing to do with installing files on a
final destination. It happens in the middle of a script
that creates an image, creates a file system on the image,
mounts it, creates device files, creates and copies other
files, creates links, finds and installs kernel modules
installs busybox, does a handful of other things, and
then umounts the file system and compresses the image.
By the time the script is finished, no destination to
copy to exists.
Sure, I could drive the entire thing with make, and lots
of things are already done in pure make. But then I would
have to start parsing configuration files as well. Not
impossible, but sometimes, it's easier to use Perl.