How would a command know which settings in your local configuration files to keep and which to replace? You may have reasons for having certain configuration options enabled or disabled on your system.
Most of the time however, the .pacnew only contains additional comments, and/or improves upon existing comments. So in those cases, you simply copy over the new comments to your existing configuration files. On account of everything else, read the documentation, consult the Wiki, or ask for help here at the forum (if it hasn’t already been addressed).
Lastly, please read through the following short essay… 