? Can’t Do Unit Conversions
Apple naturally isn’t storing Siri’s entire general knowledge database on your iPhone, so it still needs a connection to Wikipedia and other sources to answer even the most basic questions.
This sadly includes things like unit conversions. You’d expect these could be stored on your device in iOS 15, since this is precisely what many unit conversion apps do, but for whatever reason Apple hasn’t chosen to add this capability to Siri just yet.