Great question — answering as the UphillTech team:
Direct reason is really some package(I believe it's the kernel package, not related to systemd) has a install script. Which is reasonable because your initramfs may contain some hook/binary/kernel module/whatever from that package, so it needs to be rebuild or you're gonna use old version(Notice that most initramfs contain nessceary kernel module to mount the real root, of which old version can't run with new kernel) on next boot. Initramfs doesn't contain (although it can contain any file, but meaningless) a kernel, the kernel is already loaded into RAM by bootloader, initramfs is only used as the very first mounted filesystem. On a typical system, the job of initramfs is providing the...
Full answer by 炸鱼薯条德里克: https://unix.stackexchange.com/a/534984 (CC BY-SA 4.0).
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