The outcome for older kids in care is pretty shit. So eg a neglectful parent with a 12yr old and a baby, the baby might be removed and the older child left. It has fuck all to do with baby snatching though or adoptable qualities.
If the 12yr old is settled in the community, with neighbours, friends, extended family etc the ability to care for themselves to some degree, it's often a better outcome than care. Physically they might be better in care, but mentally they won't be. So for the person making the decision removal happens based on which will do the least harm long term.
Whereas the baby a) has a better outcome in care and b) has a lot lower threshold for coping with neglect.
There's also the fact that ss simply don't have the resources they would like. They would have liked to give that mother a place in a mother and baby unit to learn the skills their own upbringing didn't give them. They would have liked to offer support throughout the last 12yrs. They would have liked to offer support because of the likelihood of pnd before it happened. They would have liked to offer support when it was still mild. But their resources are so limited they can only offer support when it reaches a certain level, and even then not all the support that is needed.
There is no defence for the bad/incompetent sws. But ime the problem is that there is a ludicrously high standard before they are held accountable, so you can be a bit shit in area 1, and then move on to area 2 and be a bit shit there too and so on. And there's very little their colleagues can do about it, colleagues who find it just as wrong as the public, and meanwhile get lumped in with them.