Why does “git difftool” not open the tool directly?

There’s also an option:

difftool.prompt
  Prompt before each invocation of the diff tool.

The following command turns off the prompt globally (for all repos):

git config --global difftool.prompt false

Which is like writing in ~/.gitconfig:
(or in %HOMEDRIVE%%HOMEPATH%\.gitconfig)

[difftool]
  prompt = false