Why no knee and side airbags at the back of Volvo?

Hello Volvo friends, even after 3 years and about 80000 km I am very satisfied with my V 70 III 2.0 D. What surprised me at the time and now also with the new designs S 60, V 60: Why does a brand such as Volvo offer neither knee airbags nor side airbgs at the back – at least as a special equipment? Also my dealer did not know a reasonable answer. Your opinion on this would interest me.