You're all making Watsonian arguments to solve a Doylist problem. The simple reason why Emiya didn't have Lord Camelot in FSN is because Nasu hadn't come up with it yet.
There's something fun about that.
See, even if you toss aside this truth of Nasu's causality, you can just as easily declare that
The Throne Of Heroes itself hadn't come up with Lord Camelot yet, because there
hadn't yet been a Shielder-class servant that would need to have that specific aspect of Camelot be cast into a Noble Phantasm.
An NP to summon Camelot itself, as a fully terrestrially-manifested Marble Phantasm? Absolutely already existed on the records, with its nigh impenetrable walls being a
side note to the intended primary NP effects.
An NP to summon
just the concept of Camelot's walls, with a fraction of the mana cost for just as much defensive power as the real Camelot? Well, there is a first time for everything, and Mash would be the first-timer to do it.
Make no mistake, Gawain: in life, or in death, or in legend, or as a servant; only had the needed authority to summon the concept of Camelot as relevant to his Class Container or as part of his real life skills, but
as a subset of his skills in battle, an unnamed and unremarkable portion of the whole.