Yup its stupid af. I can adjust my game volume on the fly with the setup I have, so it's always nice to turn that shit down or mute it when I start up a game, but the fact I have to is insane.
You could prep volume mixer too, and tab out when the game launches to turn it down. Or developers could just not put loud splash/logo screens at max volume.
Yeah there's a lot of variables for audio depending on peoples setups, but having the volume default to 100% is not the correct thing for applications to do, ever.