It suppresses the rate of vigilante justice. If the courts can generally do a better job than the vigilantes themselves, then society comes out ahead.
If we were to live in a society where parents can abuse their children to death and receive nothing more than a scolding from the state to be more careful in the future, then we'd have a lot more people killing each other in retribution. And once that starts in earnest, it often forms unending blood feuds. At the very least, imprisonment for manslaughter is warranted. For the state to mete out no punishment at all would be an abdication of their duty to deliver justice so that others don't have to seek it themselves.
That's obviously a galaxy brain argument, but what's to stop someone from going Tim McVeigh over a routine miscarriage of justice?
If this was manslaughter/murder of an unrelated adult, then I could see that there could conceivably be some friends or relations inspired to vigilantism, but this thread has mainly been about manslaughter of children by parents due to incompetence.
SBS deaths are largely vastly different from intentionally abusive parents. These are accidental harms.
Also sorry but… what? You seem to be proposing that if the law doesn’t support a very specific standard of Justice that vigilantes are the logical consequence. This argument is not supported by anything and could be applied to any level of Justice.