Why are seniors with dementia or Alzheimer's placed in a double room, instead of private rooms?

And do you mean nursing homes or hospitals?

Are you asking why a rehab patient is placed in a room with a dementia patient?

Most places give you the kind of room you can afford.

Solitary confinement is far more detrimental than a roommate. When you get moved to the private room at the end of the hall, you are dying.