Why are post offices always a target for robbers in anime? I've seen quite a number of anime where the targets for robbers are mostly post offices and not banks and or other places that could have money. Is this a plot device that started somewhere or is it actually a fact that robbers actually go for post offices in Japan? If it is the latter, can someone shed some light on why post offices are a favourite target for robbers?

Some examples of anime that have post offices robbed off the top of my head are Sword Art Online and Toaru Majutsu no Index. There are probably more titles where this happens that I can't remember.
Japan's post offices are banks. And the Japanese postal banking system is nothing to sneeze at. In fact, in 2008, Japan Post Bank held more deposits than any other bank in the whole world! 

It is hence unsurprising that shows set in Japan, like Index and Sword Art Online, might feature robberies on post offices. There's a lot of money to be had there!