The main reason cpu's are run at a given safe speed is because the manufacturer does not want to risk return problems especially on a new device, also the pcb is only capable of so much heat before it twists and starts to malfunction.
Every manufacturer in the world today plays it safe and well within the specs of components and motherboards basically to make production costs as cheap as possible in order NOT to have a massive amount of returns on there hands which would be devastating to any company.
I might also like to add that by the overclocking of any cpu heat is automatically increased and as mobile phones do not have cooling fans and vents like a laptop for instance the heat generated in an overclocking situation could prove fatal in a given amount of time, days or weeks of bench testing does NOT prove stability as this can take even 6 month or years for pcb's to start malfunctioning with dry joints broken tracks intermittent joints etc etc. Stability in true form takes even years to prove not a few weeks and given the very fact that nowerdays pcb's are made to the minimum spec needed to pass protortype stage it hardly suprises me that things go very wrong because people dont take all the factors into consideration before increasing heat, HEAT is any electronics device's nightmare.