You misrepresent the intent of bitcoin along with it's intended use. Bitcoin was designed to be lost - to be eternally deflationary and for coins to fall out of circulation. Also you have to consider that the majority of coins were lost when bitcoin was worth considerably less than it is worth today.

Proper techniques to store bitcoin (local keys on a secure machine or a hardware wallet) make bitcoin mathematically impossible to steal. "Stolen" bitcoin is synonymous with "insecure" bitcoin. Properly securing bitcoin would be significantly cheaper than storing gold bars in a mattress.