How about couples with young children who found out during the last couple of years that they could live on one salary with one parent staying home. They may have had to cut down on eating out, vacations, etc. but not taking little children to daycare can make home life a lot less stressful.
That’s some of it but Americans are below population replacement levels, and with half of marriages failing that leaves only a relatively small number of intact families with kids who aren’t yet school age.