I think many people are lucky and parenthood is first time they really need support - prolonged illness, elder/disable care are less spoken about as well but probably more isolating.
What seems strange to me is how all these people with no friends just their DH. How did you actually get to meet him
I don't get how this is hard to understand.
I met DH at uni - DD1 met her Gf at school - DH workplaces were full of young people till he hit about 30-35 - so people no kids or caring responsibility up for going out - often had partners who join in bring along few of their work mate in similar position were as my work place it was long commutes for few young people and rest were settled married often kids or much older.
Plus boards here are full of people who can't meet a partner and many who end up using dating apps.
Life happens - you move, social groups break up often in response to break ups- work places change people pare off and have kids and get too busy to hang out and slowly you lose touch. it happens over years. Its not that posters are incapable of being friendly or making friends often circumstances occur and its very easy to get isolated.
I have may social older family members and friends have enriched their retirement years but when illness hits they often disappear very quickly (also outliving friends starts to happen) - it usually few exceptional neighbour and family who are around - and neighbours often don't last as if they are not careful NHS staff will try and include them in care plans - and bulk of family will often also keep distance for same reasons. MN response is euthanasia but having seen so much agism, sexism and ablism in NHS it not one that fills me with comfort.
Though I grant you some people could do more to avoid it - thinking of my own Mum here who shots down every suggestion to get more social though she got isolate in first place as Dad had years of ill health and ended up house bound.
I suspect this is a classic case of if I make it the lonely/isolated person fault then I don't have to consider this could happen to me at some point in my life.