Did your daughter in law visit you regularly before she had a baby? Did your son and daughter in law visit together more than once every few months? Did your son spend a lot of time at your house until 4 months ago?
Have you been visiting them more than you used to before they had a child?
Is your son on parental leave? Did he take more than 2 weeks parental leave?
If they are visiting less than they used to but are visiting others more it may be because you are visiting them so frequently.
To me it depends whether despite the fact that you only saw each other a couple of times per month pre pregnancy you are esentially expecting them to bring the baby to you because you want baby time, or whether they have always visited multiple times per week but have cut back since the baby was born.
I would imagine that they tbink you are seeing the baby a lot, because you visit so much, and that that is why they visit other people when they go out and about.
If you visit twice per week, and daughter in law's parents do the same, that only leaves 3 days per week to visit other people or have a day at home doing chores or just resting or shock horror go out somewhere without it being a visit.
That's even more relevant if your son is back at work, so all the visits involving him are evening or weekend.