I think a lot of people believe that the world is just brimming with support, it's just a question of "putting yourself out there" and finding it. Ergo, if you find yourself in a position where you haven't got support, it is your own fault, as you haven't tried hard enough to make friends and find the support network.
I differ in my view. Culturally, families tend to operate within their own units. People will vehemently deny this for fear of being labelled selfish, but for more cultural and evolutionary reasons it makes sense. A grandparent will look after a child for free allowing their own son or daughter to work full time: how many friends will provide that service? Trivial or ad hoc matters, possibly, but support is more than that.
When your own parents are not around - whether that is through death, them living a long way away, or a poor relationship with them - I think people underestimate how difficult that can be. But another important aspect to this is that if you have experienced your own parents dying young, or have not had brilliant parenting yourself, you are vulnerable with your own children in a way that is both isolating, no matter how much of a support network you do ultimately create for yourself, and quite upsetting, sometimes.
I remember in the early days after my parents died (not together, but sadly I lost them both young) that people were initially very kind and supportive, but that disappeared very quickly and life resumed. I have learned that relying on people for support is a risky business because for the most part, people are tied up with their own lives and their own families and supporting themselves.
I have a friend who grew up in foster care following sexual abuse
and I would say she is one of the only people who understands my anxieties about children and lack of support. It opens a can of worms, yet paradoxically a poor childhood yourself makes you more likely to want children, so who knows, really :)
It can all get a little circular and fractious but what I would say is that for the most part, no matter how many friends you make, they will not replace the support loving grandparents would provide, whether they can't be that through death, through their own behaviour or distance - doing it entirely alone is really tough.