Actually the only reason that children are more commonly abused or killed on their own homes is that there are far more children living in their own homes than in foster homes. Statistically, a child is several times more likely to be abused or killed in a foster home than in his own home.
I'm not sure if that is clear. It's like if you had 2 barrels of apples. One has 200 apples in it and 4 (2%) of them rot. The other has 10 apples in it, it gets rolled around to cause bruising, and 2 (20%) of them rot. Even though the first barrel had the "leading number of rotten apples" in it, each apple in the second barrel was 5 times more likely to rot.
Children are, on average, several times more likely to be abused or killed in foster homes than in their own homes. This has been demonstrated repeatedly and beyond a shadow of a doubt.
For this reason, the only justifiable reason for placing a child in foster care is if he can be shown to have been abused or neglected in his own home, because then that individual child is less likely to be mistreated in a foster home than in his own particular home (since his particular home has a 100% instance of abuse, and the foster home does not).
As a social institution, then, the family is much more worth preserving (in general) than the fostering institution.