Arch tries to generally avoid changing "default" behavior. Systemd doesn't automatically do that, so Arch won't ship systemd like that. Arch also generally avoids shipping with timers enabled.
Out of lazyness or out of actual usefulness to the users?
A little of column A, a little of column B. Arch is very much a distro developed for the packagers. OTOH, as someone who often has to dive into the details regardless of distro, I generally appreciate the plain-vanilla approach to arch's packaging (since I can just set the package up as opposed to undoing whatever helpful defaults and changes Debian's decided to add).
I personally (using Arch, btw) definitely prefer Arch's behaviour. If I update or install a daemon, I generally want to configure it before (re)starting it.
Restarting it and telling systemd the .service unit has been changed aren't the same thing.
Arch does daemon-reload after service file update: `(2/5) Reloading system manager configuration...`