I've seen the dynamic in couples of all ages, both childfree and families.
Sometimes it's subtle, like friends of ours in their late-30s who we often visit for dinner. They have two children under 3 years old, we are childfree. This couple share all the cooking, cleaning and parenting equally. But at dinner, I've noticed that there's a moment when the kids need to be put to bed and they have a quick discussion about who's going to do it. Every time this discussion ends with the wife offering to do it and him making absolutely no protest. I have no doubt that on normal evenings they share this duty equally, but when there's something more fun to do (i.e. chatting with friends), he gets that extra hour of freedom.
Another couple in their late 40s are childfree, he is quite blatantly the lazier, and she does absolutely everything for their dogs.
I could list dozens of other, small examples like this, in superficially modern and egalitarian relationships. Even my own, though I try to recognise and challenge these things (and not be an entitled dick myself).
There is no good reason for these imbalances to exist in these particular relationships, it's just entitlement.