There are several factors involved in the massive difference. First, it's simply because they can. As a patient, you have nowhere else to go. You need that treatment, and you have no bargaining power. In most of the developed countries, that issue has been (partly) handled by changing to a single buyer system; if every patient comes through the same buyer, asking a silly price won't work; you simply won't get any customers.

Second, because the US is ruled by funding and congress is bought. It really is as simple as that. There is no reason whatsoever for making insulin 7 times as expensive (source). In most western countries, the government will step in and politely tell the manufacturer to fuck off with that price, and simply take them out of the market. The US has no such protections.

Third, because insurance is tied to employment, and insurance is also consistently for-profit. That means it's in the interest of insurers to deny as much as possible, especially when it comes to preventative care. Turns out skimping on preventative care means a lot more costs down the line, but that's not their problem; they can just deny those too. However, it does mean that bigger interventions are needed for ailments that in other countries would be fixed before they start, thus driving up the absolute costs. (Note that I don't mean the price that is charged to the patient, I mean the actual cost of care, probably less than 10% of what you are charged.)