The Postal Office has its own police..force  1400 detectives   ..to protect us from Mail fraud...contraband...child  pornography,  etc, drug shipment.. terrorism. remember when they were mailing anthrax.
also it has prime real estate.....the lobbyists for the private mail companies and shipments are planning to take over the post  office. Online shopping has increased,,,,and when you do you get a lot of options...US post office(regular mail)   and they claim is the longest wait.....and other  options...more expensive...Fedex.....etc.