Why is money in circulation a liability of the central bank? We know that the money today is fiat currency, that it is money because the government says it's so. So when new money is printed or loaned out to the commercial banks by buying treasury bonds, government does not technically "owe" any thing. The only thing that is stopping it from creating new money every time need arises is the possibility of inflation. 

Given all of that why is still money in circulation listed as a liability of the central bank? Am I missing something here?
Balance sheets always balance, so assets equal liabilities.

Imagine a commercial bank goes to the central bank and wants cash. The central bank provides the cash, but asks for some of the commercial bank's loans (or government bonds) in return.

The central bank now has the loans (or government bonds) as assets and the cash as liabilities.

The cash is a liability, because if the commercial bank goes back to the central bank and gives back the cash, the central bank will have to give back the loans (or government bonds).

So while it's true that cash is not backed by gold, it is still backed by something. You can take your cash to the central bank, exchange it for government bonds, earn cash interest on your government bonds and then use this cash to pay your taxes. More generally cash is backed by the goods that you can purchase for it.