Unless sick people came in from traveling. But in that case again you locked back down and they went away again. Are you just supposed to be locked down forever? What happens in 6 months when you open back up and suddenly one random person gets it again? Is the solution to go right back into lock down the moment someone in the country coughs?
You can lockdown sooner or later, but if you choose later a load more people get sick and die. You'll probably have about the same amount of lockdown either way, it's just a choice of whether you want 10,000 dead or 40,000 dead, or 100,000 dead and your hospitals fucked so people die of lots of other things as well.
What perhaps the anti-lockdown brigade don't realise is the economy takes a beating either way. People stop doing stuff because they are anxious and economic activity slows down. Sick people don't work and decrease economic activity, and dead people cause economic losses, even retireees. Maybe the costs are spread differently throughout the economy with lockdown or mass infection, but we'd be haemmorhaging economic activity either way. The decision for the government is whether it wants to take an economic hit with 10,000 dead, or an economic hit with 40,000 dead. I struggle to understand why the latter is better.
Hostility to lockdown is many things. At a certain level, it's just annoyance at inconvience to oneself married with fundamental disinterest in other people dying, because "I'm all right, Jack". To some degree it's a manifestation of general fear and hatred of gubmint (one cannot help but note that the strongest anti-lockdown viwes come from those political quarters). At some level it's just the eternal psychological need for humans to impose some sort of order on a confusing and random world by acting like all the shit they can't control has a reason. Because the idea that stuff is unpredictable, amoral, unstoppable, that we are all just specks of dust blown around in a cosmic breeze, is too damn uncomfortable to process. So, when it's bad, find someone to blame: a scapegoat, a vessel to fill with the frustration and feelings of injustice, and then beat like a pinata.