Because you are installing into the current live site and as such, the script thinks you want to upgrade the current installation.
If you want to install a completely fresh copy just for testing, install it in a subfolder / subdomain or on a different server
Please read the documentation
The folder for the fresh install can't have anything in it at all
Simply unpack the zip into the empty folder and start the install
The script looks for a config file, if there isn't one it will carry out the fresh install, if it finds one, it will upgrade
I also suggest using two different databases to ensure no confusion