Dhclient has a lot of hooks where you could easily stick a script or binary to report back to systemd what the state of the network config is. But that would require systemd folks to spend time and effort on thinking about how to integrate existing software into their ecosystem, so they rather roll their own, which obviously takes much more time in total, but it fits the general NIH attitude of the whole project. Oh and it gives them the opportunity to repeat every mistake ever made regarding writing DHCP/NTP/... clients made during the past thirty years, plus some bonus RCEs.