Why are the cities of Santa Clara and San Jose California so expensive for housing?

They have no snow, no tornadoes, not much rain, and earthquakes and thunderstorms are rare. The weather is almost flawless in those places.

Supply, demand, location. The heart of Silicon Valley and no real area to grow to build acres of new housing. Population growth means increased demand, inability to create supply drives up the price. Economics 101.

The Bay area has always been expensive, taxes and people live in those areas and commute to the city.

Speculation, not enough housing no one wanting more housing for fear that their home prices would go down and they lose their retirement savings since they couldn't afford any other investments if they are paying a California mortgage. Tech companies buying properties  

In Sherman Oaks where I live is more expensive

California is generally much expensive than most other states in the US. 

Supply and demand.

Too many people, not enough houses.

maybe it just costs more to live there

There's a thing called "location". You are paying for location.