Because they fucking work.
I went through a real, strict, two month lockdown from late January to early March and a soft lockdown after. I've been happily maskless (except on public transport), bar hopping, going to the gym and movies, shopping, etc. since May.
Americans are dumb as fuck. This is compounded by the fact that the same people that oppose lockdowns will also, invariably, oppose any measures that make lockdowns unnecessary (as demonstrated in New Zealand, South Korea, Japan, and China since May). Yes, lockdowns suck. And they're unncessary, if you're willing to accept a few simple measures to isolate and prevent community spread of cases. That is:
1. Mandatory contact tracing, through apps usually that track location either by GPS or by forcing you to 'sign in' when you board public transport, enter a shop, etc. <------ This is the number 1 thing you need to prevent lockdowns because it allows you to quarantine all contacts to any new cases immediately.
2. Strict quarantine on leaving province/states/cities with local lockdowns already in place. <------------- this is number 2 so that you can prevent spread from city to city so any lockdown that becomes necessary is isolated to one location.
3. Mass free public testing. <---------- This is necessary before 1 and 2, but less necessary after community spread is largely eliminated.
3. Mandatory use of masks on public transport <------------ Only necessary in cases with high community spread.
4. Fever testing at shops to identify early risk cases <----------- Data is mixed on the efficacy of this, but all the countries that have largely eliminated coronavirus do it. How this works is that each place of business checks temperature. If you register a fever, you get sent to a fever clinic and tested for covid19.
5. Mandatory 2-week quarantine for all incoming travelers from abroad. <-------- Once local spread has been eliminated, this prevents it from popping up again.
The solution is known, it exists, it is easy to implement, and its literally just the arrogance of individuals like OP that have led to tens of thousands of unnecessary deaths. The initial lockdown should be to get a handle on community spread and identify vectors of infection. This failed in the US because of individuals like OP. Lockdown once, for real, in the next two months, put in these measures, nationwide, and watch covid 19 be gone from the US by April.
It's an easy problem to solve, and the inability of Western states to solve it is an indictment on the sort of society they have cultivated.
And before anyone says something silly like "In liberal democracies we don't believe in coercive solutions!"
Yeah, you do. Look at the top countries by share of population in prison. Come on, man. I get sent to jail for smoking a little reefer, and you can't enforce covid quarantine measures? Australia, Taiwan, south Korea, Japan and New Zealand aren't liberal democracies? You can adopt effective and temporary quarantine measures without abandoning a free society, but it requires 1) trust in government and 2) personal sacrifice - two things Western societies, these days, are completely without.