Because it is impossible to do that. See, even China did those measures that helped but ultimately they had to give up. They simply lost time unnecessarily.
No, they didn't. They implemented precisely those authoritarian measures that seem to be working at first (like lockdowns) but are neither sustainable nor even effective for any prolonged period of time (> 2 weeks).
That said, impossible for west/rest to systemically lockdown post delta/omicron, R value too high. PRC travellers already out there, whatever variant is going to go global if borders remotely porous.