Because he's with me?
She's lovely, always very pleasant to me and to DP's brother's wife. His brother doesn't have children either, although his wife has a teenage daughter, who "MIL" is very kind to, but has only known as a teenager. ("MIL" for ease, although I know she's not!). I.e. she never got to be granny to a young child.
I have grown up children and am too old to have any more, even if I wanted to. DP is a few years younger (mid 40s) and could still have had DC, if you accept that men often do with a woman a few years younger.
He says he always thought he would, in the natural order of things, but it's not something that happened for him, and it's not something that bothers him now.
MIL is a very mumsy young older lady, with bags of energy and loves fussing over people. She'd have made a great gran.
Obviously there's nothing to be done about it, but I do wonder if she wishes things were different.