Because they don't exist.
WHAT system-on-a-chip?
http://www.wired.com/gadgetlab/2009/04/apple-quietly-recruits-chip-designers-for-in-house-cpus/
http://www.theregister.co.uk/2009/04/30/iphone_soc/
http://www.appleinsider.com/articles/09/05/21/apples_future_iphones_may_leverage_arms_v7_cortex_designs.html
apple hasn't been hiring all those individuals for nothing...