Because i can install vundle with a single line in my setup script (already have a "provisioning" kinda script for each time i fire off a machine).
I dont have to update that script for new plugins, its a single "Vundle user/repo" in the vundle.rc file. Vundle then installs it, and handles it.
Could it be scripted, sure... but its much more readable in the vundle form (or other managers for vim), and its also in a more logical place... and its portable, even to a windows machine... its kinda a separation of concerns kinda thing...