That's a reasonable question with several answers.
One is that US healthcare cost inflation is very high. The average family premium in 1999 was $6k, it is now $27k, for an annual cost increase of 6.1% per year. The long term rate of productivity increase is much lower than that, at only 2.1% per year.
So costs have just risen a lot more than productivity has.
Another reason is that productivity increases aren't evenly distributed. Most productivity growth has been in other sectors, primarily oil+gas and tech i.e. sectors dominated by men who aggressively automate. Healthcare has seen no increase in worker productivity for decades:
Output is up, but only because of more hours worked. And much of that output is growth of administrative overhead, not actual healthcare as most people perceive it.
Soaring demand + zero productivity growth + cost inflation 3x higher than inflation + no political will to control costs = a death spiral in which the lowest risk decide to go it alone and drop out, leaving ever higher premiums for the rest.
