There are many examples of for-profit systems that don’t have this problem, it’s really the heavily regulated for-profit systems that have this “cost disease” issue. It seems to happen whenever there isn’t a transparent market, like the tuition price of a university, the cost of your surgery, or the cost the government will pay for some infrastructure. The buyer doesn’t know what they’ll pay or what product they’ll get for it, so it’s basically not a free market at its most fundamental level.

If it's expensive and in a for profit system, why aren't competitors on the supply side undercutting each other to increase sales?

Not every for-profit system is this way so it doesn't fully explain it.

Every country has for-profit elements in their healthcare system. The USA is uniquely dysfunctional in its governance. There is regulatory capture at every level.