@SJ7 Probably has to do with their living situation, they seem to be in an apartment, and many of them don't allow pets.
So, if they were to get a dog, they'd probably have to get a house first.
And the best thing to motivate them to upgrade to a house, is a lack of space in their apartment, due to another child on the way.
I'm halfway convinced this series is bankrolled by Abe, so this seems like the logical route.
A house, with two kids and a dog. How big is Japan on white picket fences?