I don't think that's part of the reason either. Sure, weather might be a factor for employee comfort, but most certainly not for a startup friendly environment. Else why is Stockholm or Copenhagen a bigger startup hub than Barcelona or Rome? Why are all the French startups in Paris and not Marseille, or the German startups in Berlin and not Munich? From even a non-startup perspective: why is Milan a bigger hub for corporate than Napoli or Rome (don't know how true the last bit is, but certainly felt that way)?

There are also historical reasons. Northern italy has long been more industrialized than the south, going back centuries to before there was a modern unified Italy. Paris has over twice the population of Marseille and way more universities which feed the job pool. Scandinavian countries have stronger social safety nets that foster more growth than their comparatively lean budgedted peers in southern europe.

Compared to total land mass, sure. But (as that map clearly shows) most Mediterranean climate is not in California. If it's just about the climate, there are plenty of options.

...or more likely, the weather is not a critical issue.

Why can't the weather be a critical issue that only works in combination with the other factors?

My intuition says that Silicon Valley wouldn't be possible in a beach town nor in a place that's too cold or too hot, simply because of how much the weather would distract you from work. The great thing about the weather in the Valley is that it's pleasantly bland (or blandly pleasant?).

But I think it's very likely that the weather doesn't matter at all if you don't already have some equivalent of the Stanford-Berkeley nexus and California labor law and a lot of very rich people willing to make high-risk investments.