I think people are also a bit in denial about their own role in this. It's comfortable and nice to be in a couple, and if you hung with other couples or as family groups, that's comfortable and nice too. Did you, before you were widowed, invite that single mum with three kids whose just been divorced? Did you have your friends over who don't have kids but love to be invited to the BBQ? Did you reach out to people who were grieving, perhaps lost a parent or had a miscarriage and make sure they were invited to everything? Did you know any widows or widowers and include them? If you did, then you will still have lots of single, divorced and widowed friends to hang out with. If actually you kind of hung with couples all the time, and those people weren't part of your friendship groups, however nice it was, then perhaps it's just a consequence of our society's pressure to couple up and be heteronormative (count your lesbian/gay friends as well)?
It is horrible when fear of death and grief drives people away from you, we are crap at talking about this stuff in our society, not all societies are like this (my Mexican friend says it's normal to chat about their dead relatives and ghosts over coffee!)
But- we all have to own that a little bit as well, and think through whether we have always been a great friend to single or widowed people in the past (like inviting them on holidays) and whether we also unwittingly didn't invite along people who didn't quite fit with the vibe or the coupled-up or close nuclear family nature of the crowd?
I know I could have done better, it can't just be everyone else is selfish and crap, we are just products of that culture and can make an effort from this point out.