It's that they're extremely hard to help

From The two most important numbers in American health care by Ezra Klein.

The two most important numbers in American health care are 5 and 50. Five percent of people account for about 50 percent of the health system's spending.

[snip]

What was amazing about the conference is that though these groups serve the sickest people in the country, the discussion had almost nothing to do with treating actual health problems. It was about treating everything else.

All of these organizations have learned the same lesson: The problem with the 5 percent is not simply that they're extremely sick. It's that they're extremely hard to help.

These people aren't just ill. They're poor. They have mental health problems. They're dependent on wheelchairs. They have dementia or brain damage. They live in unsafe homes. They don't have cars. They're agoraphobic. They're worn down by chronic pain. They're stubborn. They're flaky. They're angry and they're fearful. They're bad at talking to authority and working through bureaucracies. They're wary of the medical system and cowed by doctors who don't seem to have time for them. They're locked into bad habits and used to bad environments.

Our health-care system can deal with very sick. Our health-care system is arguably the best in the world at dealing with the very sick. What we're bad at is dealing with everything that happens outside the hospital -- all the things that keep making these people very sick. And so long as all those other things go unfixed, these people keep getting sick, and they keep racking up huge bills -- not to mention facing enormous suffering.

Very analogous to the causal density and causal complexity which plague education outcomes. It is not so much what happens within schools that determines outcomes as what is happening outside of school. And when you venture into those wilds, things get very complex very quickly.

We want simple answers to complex issues and those are very rare. Well, simple answers aren't rare per se. Simple answers that work are rare.