Despite their corporate headquarters physically in the Silicon Valley, many Californian companies incorporated in the state of Delaware seeking better protections against hostile take over. For example, SGI was incorporated in California in 1981, but it reincorporated in Delaware in 1990. This is mainly due to differences in Corporate Business laws in each state.
A similar strategy is used for financial institutions. Many U.S. States have laws limiting the amount of interest a corporation can charge, but Federal law allows corporations to 'import' these laws from their home state. Delaware (amongst others) has relatively lax interest laws, hence the preponderance of credit card companies in the state.