Babies, just like adults, are extremely social animals. And they absolutely need to interact with a bunch of other people their age, even more than us. Daily, and for a long period of time. An hour at the park doesn't cut it, and being all day with a sibling doesn't either.

So beyond everyone going back to a Neolithic way of life and living in a bunch of straw teepees all bundled close together, daycare is the best solution I've found to this need.

Just as an example, my oldest has been besties with another kid since they were both 7 months old.