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
You just posted that you've upgraded your site (in a different post), so which one has the problem ?
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