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.

As a counter-example, neither of my kids really acknowledged other kids in any way at that age (and other infants ignored them right back). A quick Internet search suggests it's normal for them to not interact with other kids until after 12+ months. This was a point of contention with my wife and MIL because my MIL would complain we weren't "socializing" our oldest enough when she was an infant despite clearly never having looked up anything about childhood development.

That and we did take her out all the time. She just wasn't in daycare. The thing about stay-at-home parents is they don't literally stay at home all day.