Normal banking systems can have the same problem when they, too, have lax security. Witness the $100 million heist from the Bangladesh account at the NY Fed.
http://finance.yahoo.com/news/hackers-steal--100-million-fro...
http://finance.yahoo.com/news/hackers-steal--100-million-fro...
So, people should trust or build on existing system if they care about their stuff disappearing or being stolen. There's mitigations that work for that situation for the common case. Unlike Bitcoin and its exchanges.