Eventually the kernel has to be updated, which you want to reboot for. Plus ensuring that the entire userland is running the same new versions of base system libraries isn't much less disruptive than simply rebooting.

Anyway, ChromeOS swaps the system and kernel between 4 partitions [1] - automatic update effectively installs a new system into the system partitions you're not booted from, then tells the boot loader to boot from the other ones next reboot.

[1] https://www.chromium.org/chromium-os/chromiumos-design-docs/...