The reason is that Fugitive only defines its commands for buffers whose files are under Git version control. Precisely, the code in plugin/fugitive.vim
only sets up autocmds that detect files under Git control, and only then defines buffer-local commands.
So at least you need to pass a Git-controlled file to your Vim invocation. If that still doesn't work, try explicitly triggering the detection via
:doautocmd User Fugitive