Multiple placements are the result of 2 things:

Non-permanent placements (most obviously crisis care), which by itself happens to 20-30% of children in care. A placement in a youth institution is a per-6-month assignment by a judge and therefore obviously very unstable in essentially all cases. Even when they are stable, because of the 6 month time limit, after which a decision the child has zero possibility to influence might change their environment 100%, do you really think this will be experienced as stability and is correctly classified as "no change"?

Additionally, in clinic or some forms of foster care one might even say that given the rate of changes in the group (and personnel) even if the child remains in the same location, everyone around them changes in something like 8 months, every 8 months (because 10 children per group, every one has 6 month placement, so you can expect some change every 2 weeks). On top of that, personnel changes are very common in all youth institutions. So even in the case of a stable placement a youth institution will not be experienced as anything remotely resembling stability by a child living there.

The viewpoint of the child is obviously what matters here.

Breakdowns of placements (ie. foster parents/institution requiring child to be removed immediately, without waiting for placement to end): 20% to 65% of children, depending on age and gender.

https://www.researchgate.net/figure/Rate-of-breakdown-by-age...

Is it really so controversial for you that multiple changes of caregivers and environment are experienced by almost all children in state care? And that constant large changes of environment are experienced by a large subset of these children?

I did not say the worst possible form of reactive attachment disorder is the expected outcome in 60% of cases, some form of attachment disorder is expected in 60% of cases. And granted, you can certainly make a valid argument that this is at least partially because every DSM revision loosens the definition of attachment disorder, which is also seen in other diagnoses and results in absurd levels of comorbidity.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6097443/

The real problem that everyone does agree on is that reactive attachment disorder sharply rises when social services intervene in the lives of children, irrespective of the reason. Even abused children have sharply lower incidence of attachment problems than children in clinic or foster care.

This is controversial, that's certainly true. Why? Hmmm, let's ask ourselves ... why might a very good reason to avoid interference in families by social services that has historically been used to deny almost all interference (and with that came the consequence that it robbed many social workers of a job and greatly limited the size of the sector) be controversial? I wonder why that might happen...

> Also nonsensically false that "mars bars instead of sandwiches" would get any intervention.

This is seen as neglect and will most definitely cause intervention by social services. Hopefully placement is rare, but the issue is that if you attract the attention of social services, a child getting an out-of-home placement is greatly dependent on the child services investigator as the first linked study pointed out, not on the child, the parent, or what is actually going on. Just for that reason alone it will happen.