Like a lot of things in KDE, this is configurable: Settings -> Configure Kate -> Session (from the sidebar) -> Uncheck "Show welcome view for new windows" under "Application Startup/Shutdown Behavior"
Having a gui for settings helps with not having to memorize in what forsaken directory an application keeps its config files, because apparently that's not standard. Is it in .config? Or maybe .<app name>? Maybe /etc/thing?