There is high quality long term evidence that nursery access does make a big difference to children from disadvantaged homes, whereas the other stuff suggested to be funded instead can be quite hit or miss.
The main problem with poor parenting/neglect is that the reasons this happens are hugely complex. It's not as simple as giving people advice (most of the time) and to be totally honest, I would be extremely surprised if genuinely nobody has given her advice. My experience is usually that the person is given advice but they hear it as criticism. Or it happens as part of a larger conversation, and they remember other parts of the conversation but miss/forget the advice. Or they dismiss it, thinking it doesn't apply to them and then forget the interaction. Or they do take it on board but because of various (usually complex) reasons, they are unable to implement it for very long or at all. For similarly complex reasons they may be unreliable narrators. Seeming unaware of basic parenting ideas does not mean that nobody has ever told her, and on some level, finding out what constitutes reasonable parenting is also part of the responsibility of a parent.
Nursery will give children experience of a structured, routine-led environment, exposure to other children and adults for social development and experiences, plenty of non-screen related stimulation and time to play and explore with different toys/environments/experiences. They will be read to and counted with and spoken with and listened to. They will be offered at least one solid meal. These things are all hugely important for children and most children DO get these things at home, thankfully. But for the children who don't get these things at home, the chance to experience them at nursery gives them a huge boost. The difference between getting some experience of these things vs a lot of experience of these things is much smaller than the difference between getting some experience vs no/very little experience. That difference is huge.
For some parents unfortunately you can counsel and advise them as much as you want but they won't change their behaviour. Most people, even if they have very little experience with children or experienced neglect in their own childhood etc know that there are parenting books, websites, support groups and will use these if they need to.
I do know what you mean about supporting the whole family. In an ideal world this would be better, and I think it is a great shame that it is not possible or practical to offer this support but realistically, I think you are naive about the sheer amount of support and time and input which would be necessary to make a real difference. A meeting and plan made with a social worker is unlikely to be anywhere near enough. It might help in some cases, but it probably needs much more intensive 1:1 daily support, for periods of weeks at a time, over years. And even then, it might not be enough.
I don't know if you ever saw the old Supernanny programmes. I realise they are programmes made for entertainment, and TBH I haven't ever been the biggest fan, but the reason I mention them is that she did often implement this kind of advice - looking at things like bedtimes and parents spending quality time with their kids. I watched an interview with her recently where she talked a bit about the behind the scenes on the show and two things I hadn't realised was that she actually spent 200 hours with each family, and that she had veto power over which families she worked with and she tried to select for people who actually wanted help (rather than wanting to be on TV). Obviously it's edited down to show the most dramatic angle and it's very heavy on the "Super Nanny cracks the naughtiest kids ever!!" but watching some of the older programmes through the lens of this you can see that what she is doing is really parent training, not child training. And yet in every programme, even when the parents genuinely want and invite the input, even though they have had someone follow them round 24/7 making observations and providing coaching of their parenting right there in the moment, they always fall out of the routine, they always get things wrong thinking they are getting it right, they always miss really crucial steps, it generally takes a lot more coaching and support to get things to a better place, probably realistically much more than the 200 hours or whatever it was.
It's really hard for people to change habits. It takes a lot more than some instructions and advice. The person has to really want to change and believe in the value of the change and understand why it will help, and possibly also understand why they were doing it differently before. For parenting they also have to be prepared to deal with pushback from children who tend to like the idea of getting unlimited screen time and junk food and no bedtimes, and will fight changes like this even if once they are used to the new rules they are ultimately happier. And all of this while the problems which were underlying their difficulties in managing basic parenting responsibilities in the first place probably haven't gone away or been addressed. And then remember this is someone coming in who has not been invited, but an authority, saying you must do this/that/the other. Not all parents are going to agree with the ideas that the social worker has about good parenting. Even amongst good, involved, educated parents there can be raging debates about topics like co-sleeping vs sleep training, for example.
OTOH you can fund childcare and give children a few hours' experience each day of the critical things that they need. It works, immediately, it provides measurable benefit to the children whether the parents get their shit together or not. Parents accept it because it is not seen as a direct criticism of them. It's absolutely a sticking plaster, but the metaphor for what is needed is complicated reconstructive surgery, and that isn't realistically possible, so a sticking plaster will have to do.