A lot of people do not understand what systemd-resolved is, from an architectural standpoint, and because of that do not ask this, which is actually the right question. Yes, running one's own resolving proxy DNS server is widely regarded, by those in the know anyway, as the preferable course of action to using an external service.
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.
