Why wouldn't automatically starting a simple local recursive resolver be a better fallback than consuming a public one?
The answer is (I suspect) that the systemd people haven't supplied a resolving proxy DNS server of their own to be run, and don't want to tie themselves to running a particular third-party's software. So they tie themselves to a particular third-party's service instead. systemd-resolved has to have somewhere to forward to by default, it being architecturally a purely forwarding proxy DNS server.