It's actually the opposite. When someone doesn't get the health care they need and their condition becomes critical, they end up at the ER, costing many times more. A doctor's visit to get a prescription to reduce high cholesterol or blood pressure is a hundred times cheaper than calling an ambulance to deliver that person to an ER for emergency heart surgery. Prescribing antibiotics for pneumonia at a doctor's office is ten times cheaper than doing the same at the ER.

It's actually not the opposite. Giving people access to early care doesn't reduce health care costs. Yes, you save some money by avoiding the critical illness treatment. But you end up spending more money on the preventative care. They also tend to live longer with whatever chronic diseases they do develop. And those things tend to balance out, and you don't save any money. That's not a reason not to do it, of course - the people getting the preventative care are healthier, and you want to make sure they get their cholesterol and BP meds. But even though it's a hundred times cheaper to give them the meds than take them in an ambulance, when you give the meds to 100 people for every ambulance trip you avoid (ambulance trip being a metaphor for all the crisis care), you don't save any money:

These statements [that you save money by giving people access to preventative care] convey the message that substantial resources can be saved through prevention. Although some preventive measures do save money, the vast majority reviewed in the health economics literature do not.

https://www.nejm.org/doi/full/10.1056/NEJMp0708558

There are thirty different ideas there about how to provide UHC. Some better, some worse. A lot to learn from, a lot of possibilities to come up with a workable system.

Yes, lots of possibilities to look at how to come up with a workable system - but no opportunities to see how to come up with a workable system starting from 18% of GDP (or wherever we are these days). No opportunities to see how to come up with a workable system in a political system where special interest groups have many structural advantages to block legislation. Etc.

As you point out, every other country has done this in the last century. But we haven't. Businesses are greedy everywhere; people are reticent to have their rice bowls dinged everywhere. So why haven't we already done this? What is it about our system that has kept us from doing this?

That's what you need to understand if you want to figure out what's going on. And the answer is pretty clear, based on the failures in Vermont and other places. We've allowed the health care system to grow to the point where it is no longer possible to switch without materially affecting a lot of people such that they are materially worse after the transition. We've got a political system that across the board allows special interest groups (which includes unions, retirees, medical providers, and others) to effectively exercise power in ways that do not occur in other systems.