Welcome to the 21st century, the New Order.
Forget all that legal mumbo jumbo you may have read back in law school in the 1960s about commercial code. Its all gone now. Now we have Check 21 and the Patriot Act !!!
Basically what this means is that because some Arab fanatics burned down the World Trade Center, the US government and its allied civilian banking company henchmen now have total control and dictatorship over "your" money, which is no longer really money, but more like a "credit" to your account with THEM which they can do with what they want. Here are some of the many consequences of the two aforementioned acts:
(1) You can no longer sue a bank for mishandling your money
(2) All your banking transaction information is the joint property of the bank, its "affiliates" and the US Treasury
(3) You can no longer conduct private monetary transactions with other people using a bank as your agent; you can only request that a bank execute an unsecured transaction on your behalf and the bank has total control over that transaction and the terms on which occurs; you have no say over these terms and you cannot sue a bank over any financial tort on you for any reason.
(4) All banks are required to spy on you, report any "suspicious" actions on your part, develop and run special software to detect these "suspicious actions", and send their employees to government-run educational courses where they are taught to spy on customers, how to report suspicious customers and how to seize money and safe deposit boxes from customers when the government orders them to do so.
(5) All banks are required to positively identify everyone who has a bank account or safe deposit box and report all their accounts to the government.
(6) No transactions can be done anonymously. All parties to every banking transaction must be identified and recorded.
So, from the above it should be clear to (if you are a lawyer) why no endorsement is present. That is because your check is not a negotiable instrument anymore, it is merely a request to the bank to transfer funds to the Treasury. The Treasury does not need to "endorse" anything. In fact, legally speaking, the Treasury could simply order your bank to empty your account into theirs, and they actually do this all the time to people they are "investigating" for supposed crimes.
You don't need to endorse checks you receive either because, as I said above, the check is no longer a negotiable instrument. Banks still have people do it, but it is just a pro forma habit from the old days. Since you can't sue the bank, the endorsement is pretty meaningless because it cannot be challenged in court anyway. You could probably just write "X" there and they would deposit it.