A while back, a colleague was faced with an author who wanted to say the Silicon Valley rather than just Silicon Valley because, after all, we say the Ottawa Valley.
But the Ottawa Valley is the Ottawa Valley because it’s the valley of the Ottawa River. I grew up in the Bow Valley, so called because it was the valley of the Bow River. There is no Death River and no Silicon River; the names Death Valley and Silicon Valley are not descriptive formations based on some geographic feature. A valley doesn’t need to have a river to get a “the,” but the “the” generally indicates a central geographic feature contained by the valley, and that geographic feature is the focal detail, not the valley – the valley is presented as a surrounding attribute.
On the other hand, places named after some feature or associated quality or thing such that the place, not the associated thing, is central (and the associated thing is an attribute) normally don’t take “the” – Moraine Lake, Rainbow Falls, Happy Valley, Cougar Mountain, etc. So if it’s a valley first, it’s likely to be X Valley, whereas if it’s a river or whatever first and the valley is an attribute of it – if it’s the valley of the X – then it may be the X Valley.
But the main reason that Silicon Valley doesn’t take a the is just because it doesn’t. Never mind arguing from reasoning; place names are varied enough that exceptions can typically be found for any rule. Place names adhere to what is actually officially and commonly used for the place name, and it is not officially or commonly standard to say or write the Silicon Valley. It’s like saying the New York or the Vancouver Island.