Nobody ask me ;(
You can tell them that you were looking for an operating system with far fewer distractions and a minimalist feel, and that GrapheneOS met your needs. you can also respond that since most of us store personal data on these devices, having a secure device and operating system that respects your privacy gives you peace of mind. If the person responds that they don't care because they have nothing to hide, you can respond that prevention is better than cure, and let it go if the person isn't convinced. I suggest avoiding saying that it's about avoiding Google and data collection altogether. Instead, show them that you can use Google services like everyone else and that your battery can last longer because Google services are not privileged. Show them that you can install it in private space so that services don't run constantly in the background, which should further reduce battery consumption.