I remember a line from a movie I saw years ago: "It may be that you learned everything you know from me, but I didn't teach you everything _I_ know." Similarly, it may be that everything we saw at the oil rig was Enclave, but it wasn't everything that the Enclave had. The Oil Rig was the Enclave HQ, located as Eden said, on the West Coast. But from "the decades following the war" to 2077 is a couple of centuries. And the Enclave was, after all, what remained of the US Government. If anyone was in a position to know where EVERY government-controlled installation was located, and ALL of the passcodes needed to enter those facilities, and how to tap in to the government communications channels that linked every base and facility, it would be the Enclave. Put yourself in their shoes: what would you do during those intervening years? It seems obvious to me that when the radiation dropped low enough, seek out and search bases and installations to see if they were still usable.
It may have been that the Oil Rig had some limited R&D capability, but it certainly did NOT have the capability of manufacturing those high-tech Power Armor suits that were so prevalent in FO2 and FO3. And that kind of manufacturing requires A LOT of raw materials to produce that many units. The Vertibirds may very well just been taken out of mothballs, but those suits were definitely higher tech than anything that was available before the war. And those kinds of manufacturing and resource gathering facilities were clearly not in evidence at either the Oil Rig or Navarro. So if not there, where? And if there are other sites not mentioned in FO2 or FO3, then why not A LOT of other sites, all ruled from the Oil Rig? (Until it went kaboom.)
As for why the Enclave personnel fought to the end in Broken Steel, well, they were soldiers. And soldiers that perceived themselves as patriots as well. They followed their orders, fighting to the end against people they perceived as little more than neo-barbarians. While they fought on, the Enclave High Command was trying to sort out who would be in overall command (someone that unified rather than caused schisms) and formulating plans to accomplish their long-term goal of resurrecting the US of A -- provided it was a US of A that the Enclave controlled.
[Historical esoterica: If you ever had the chance to see the translated archives of the USSR concerning orders that were issued to the forces inside Stalingrad, you would see how it is that the Enclave soldiers would literally die fighting "to the last man, to the last bullet." Surrender and retreats were simply NOT options. In contrast, the German's "will fight to the last man, the last bullet" wasn't adhered to quite so well. Between the two sides, we can see the full range of human commitment to a goal. (Whether that commitment was self-generated, or "motivated" by The Powers That Be.)]