You can also take them to events at the public libraries and other places, at least in my area. They're often called "Library Story Times" and they're free where we live. That's what my spouse does. There's a very wide spectrum of social activities available even for kids of stay-at-home parents. She will often get together with other parents and let our son socialize with their children too.

That’s fair and I’m sure your spouse covers the need for socialization, however these options aren’t available everywhere and not all parents are going to take their kids for these events every day. Having a venue that you don’t need to plan for 5x times a week is always going to be a great default.