For decades, the infrastructure needed to keep your public-facing websites online has had a relatively simple design. After all, it was all contained in one or a few datacenters under your control, or under the control of a contractor who did your bidding where network architecture was concerned.
Requests came in, DNS resolved them, the correct server or pool of servers handled them. There was security, and sometimes a few other steps in between, but that still didn’t change that it was a pretty simplistic architecture. Even after virtualization took hold in the datacenter, the servers were still the same, just where they sat became more fluid.