You should not place too mucgh reliance on GDP figures which only measure the amount of money churning around in an economy rather than the economic health of that economy.
Look at that Business Insider link again, you'll see Italy and Brazil at 9 & 10 in the list of economies yet both those nations are effectively bankrupt.
If California seceded, its bonds, no longer underwritten by the US$, would be worthless. The state would be unable to fund its deficit and would quickly become a failed state.
Recent growth in the Californian economy has been largely due to stimulus money pumped in by the federal government.
And having the bulk of the world's big tech companies is not really an advantage when what these corporations are most famous for is not their skill at technological projects but their skill at offshoring profits.