infinitieunique Because the GrapheneOS is a small development team and prioritizes features that benefit the security and privacy of the devices they offer. An important aspect of that is being able to ship Android and Pixel security updates, and in particular major Android version updates, promptly and reliably.
Making changes to AOSP such as to the battery usage screen that you are proposing here would not only divert resources away from working on the next innovative privacy and security features, but it would also significantly increase the work required to release major Android version updates quickly on GrapheneOS. That would ultimately lead to slower updates.
Such an outcome would be counter to the aims of the GrapheneOS project, and is avoided by aligning GrapheneOS closely to AOSP.
That said, there are still a great array of changes that GrapheneOS makes to AOSP to improve privacy and security, and those are listed on the website: https://grapheneos.org/features.
The GrapheneOS developers set themselves high standards not only for the immediate privacy and security benefits of any new features, but also for their resilience to new Android version levels, and eventually also for new devices.
I am not part of the development team, but I hope that helps to provide a little insight into how development decisions that are made.