Zero-COVID is a key policy of Xi Jinping. He is using it as a signature of his administration. Xi's prestige is strongly tied to the success of the policy. It had to be enforced until after the All Party Congress. Now that it is confirmed as policy, any sudden reversal would be seen as Xi admitting he was wrong. Chances are good he would be out of office quite rapidly.
Further, it is quite obvious that the political authorities enjoy having the power that goes with the policy. For example, nearly all Chinese citizens, particularly in large cities, have QR codes on their phones. The local authorities can turn those codes red with a click of a button. When your QR code is red you are required to stay home until it turns green. You can't shop for food, get on the subway, enter most office buildings, etc., without a green QR code. The same app maps the location of the phone 24/7, thus making it possible to know who was at any given location at any given time. Persons involved in activity the authorities don't like get their QR codes turned red.
Things like complaining about being locked down for 100 days. Or complaining that the fire doors on an apartment building being welded shut. Or complaining about the lockdown being much more harshly enforced on Shanghai, an area where Xi Jinping's political rivals have their strongest support.
Power corrupts. The power over QR code corrupts digitally.
At this point, lockdowns are manifestly not helping with COVID. The Omicron variant is nearly as contagious as measles. An infected person walks through a room. Then, 10 minutes later, somebody else walks through, and gets infected. No measure that could be performed by the public will stop the spread.
Another aspect of the current situation is momentum of cooperation. Local authorities want to be ostentatiously seen to be helpful to the regime. This is a pattern at least as old as the Cultural Revolution. (Though, to be fair, it is present in stories of various emperors, for many centuries, both in and outside China. And, to be fair on that, yes it does compare Xi Jinping to the excesses of historical tyrants.) Thus, they are zealous in their own support and following of the lockdown, and in recruiting assistance. And in taking actions that support the policy and push compliance, with little or no thought to whether they actually achieve anything with regard to slowing COVID.