"They have all done well, children and grandchildren, seem happy and finding their place in the world,"
That's good.
People don't 'do well' without putting in a lot of effort - and effort takes time, so you already know they are busy people.
How do you want them to include you?
"I have a busy life and friends"
And how does that look from the outside - to family for instance?
'Oh, Mum's always too busy, so won't want to come with us.'
'Mum has her own friends and the kids are too noisy, they might annnoy her.'
I remember when my darling grand-daughter got to the age when she no longer wanted to spend the weekends with me doing crafts, cooking, games and walking in our ocal woods as she used to enjoy.
She was full on with all her extra curricular stuff, constantly studying and occasional partying. She wanted to 'do well' - and did.
But there was no place in that for a Grandma. All perfectly normal. Don't take it personally.
Have you invited them to be part of something in your life?