With my parents, we take turns. When it is just my mom, me and the kids, my mom pays more often. When it is MIL, we pay because she is broke, like borrow money from you children and only pay them back sometimes broke. DH learned his lesson a long time ago, but she still does it to his siblings.
Our parents. My parents are in a much better place financially than they were when I was growing up (we never ate at restaurants) so I think they like being able to treat. It isn't frequent we all go out anyway.
My ILs treat because they want to, I guess. We don't see them a lot.
ETA: in both cases they are the ones who invite us out to dinner. We do not initiate dinner plans ever.
My parents usually pay for a random dinner out. For occasion, noncelebrating party pays (we pay for parent's bday, they pay if going to celebrate kids' last day of school, etc)
My dad lives three time zones away. I don't see him much, and he usually pays. Maybe one meal per visit, I pay.
MIL & SFIL always pay when we go out. We've started getting them to let us pay for our drinks since they don't. Maybe someday they'll let us pay for dessert.
We pay when we go out with mom, but that's different.