because staff have access to them. When I was in the Brazil MTC everyone on our floor had our safes "broken in to". I had $500 cash stolen along with my digital camera. I lost every single picture I had of my 2 month stay (still depressing to me). My companion and district friends all had theirs stolen too so we have no recorded memories of our MTC experience.
If you think that's bad, I also had a "break in" into my locker in the provo temple. And they stole my scriptures!
Apparently, no place is safe.