As a Letter Carrier for 32 years,I can tell you there are any number of reasons why it takes more time for some things to get to where they need to be. Yes, we are human and we do make mistakes. However, most of the mail is put through machines, and the ONLY time a human even reads it, is when it gets to the letter carrier to be delivered. Most of our letters we don't even TOUCH until we are out on the street delivering them. If the machine reads the address wrong, it can take DAYS to get it corrected. I had a letter recently that belonged in the same city, but the street was wrong, it showed up in my mail (the stuff I don't touch until I'm on the street) 4 days in a row.. and I did everything I could to make sure it didn't keep happening, and still I kept getting it. Why, because machines can't think... they scan the address and send it where it thinks it needs to go. If it is wrong, there is no way to fix it until a Letter Carrier sees that letter, and even then, it doesn't always get fixed.
Also, the percentage of mail that is late, is really very small. I know that isn't much consolation when it's YOUR letter, but on average, I get 1600 pieces of mail in that stuff I don't touch until I am delivering each day. In my station there are 50 routes, in my city, there are 11 stations. If each route in each station got the same number of letters on average that is over 880,000 letters IN ONE DAY, that is being delivered in my city... now multiply that by every other station, town or city nationwide and you are talking BILLIONS of pieces of mail every day. In that 1600 pieces of mail that I don't touch until I'm out delivering, I may get 15 to 20 pieces that don't belong on my route each day which is more that I would like to see, but in the greater scheme of things, really isn't much at all.
Even in the Postal Service, they are cutting back on positions. Parcels get worked, when they have the clerks to do the work. If they are Priority, they SHOULD get worked first, then Parcel Post or Media mail would get worked if they still had time. If they don't have time, then those parcels wait until the next day.. and again, priority would get worked first, then the parcel post from the day before should get work, before the parcel post from that day. If someone calls in sick, they don't have any one they can call to replace them, they just have to work with one (or more) positions short.. and do the best they can.
Even in the Postal Service we have people that take great pride in their jobs, and those that just show up to collect a paycheck, and don't care about the quality of work they do. Unfortunately, those that don't take pride in their jobs, make all of us look bad.