I asked my parents exactly this.

They said something like

It was possible to live on one salary, so your mother could be a full time stay at home mom. This gave us the energy to be able to follow other passions

Basically 2 people with 2 full-time jobs between them have more energy than 2 people with 3 full time jobs between them.

Mom could volunteer and join clubs while I was at school. Dad could do marathons on the weekend. We could do family activities like camping or skiing because they weren't exhausted by also having to do another 40 hours of work per week between them.