Neither the California Labor Code nor the federal Fair Labor Standards Act require employers to offer severance agreements to departing employees. Instead, severance agreements are provided by employers to accomplish a specific goal.
Severance is never a requirement of any employer unless you have a signed employment agreement stating otherwise, or, it is a written policy of the company.
You do not get severance if you quit. Nobody is automatically entitled to any severance legally, ever, unless you were hired under a contract such as a 1099 employee and you have severance written into your agreement. Standard W-2 employees usually do not get severance.