Silicon Valley just means a concentration of influential and powerful actors including venture capitalists, executives, entrepreneurs who decide the fate of the technology industry. Basically the elites of the XYZ sector.

Same can be applied for DC (politics/military), NY (finance), LA (entertainment).

SV was around since the 60-90s but didn't get much attention until beginning of this millenium due to the huge value creation and control they had over the US economy. They just happen to be relevant in recent times. 100s of years ago it was the railroad and oil conglomerates, 1000s of years ago kings and feudal lords. So there is always a powerful influential class who controls the strings -- its a feature of human society.