Practice, with guidance is superior to practice without guidance. Homeschooling doesn't mean isolation in the average case. You get a lot of practice as a homeschooler. The primary difference is that your practice is both with other adults and children while supervised and also modeled directly to you in homeschooling by other adults while public school is primarily unsupervised and lacking in a modeled behavior to observe.

The number one thing people would comment to my parents about me was that I was so comfortable socially in adult conversations and environments. I wasn't even in high school yet. I had adult level social skills by age 12. I didn't learn how to interact with people from other kids who had no idea how to either. I learned it from my parents and practiced what I learned with both other children and also adults. I'm only anecdotal evidence but a number of studies have backed up my own experience. A few links I had on hand can be found here.

* Medlin, R. G. (2013). Homeschooling and the question of socialization revisited. Peabody Journal of Education, 88(3), 284–297. https://www.stetson.edu/artsci/psychology/media/medlin-socia...

* Shyers, L. E. (1992). A comparison of social adjustment between home and traditionally schooled students. Home School Researcher https://archive.org/details/comparisonofsoci00shye

* Taylor, J. W. (1986). Self-concept in home-schooling children. (Doctoral Dissertation). Ann Arbor, MI: UMI Dissertation Services. https://digitalcommons.andrews.edu/dissertations/726/

You don't just stay home with your siblings. A major factor of how homeschooling works is homeschoolers have local organizations or co-ops where they do things together.

And, yes, this is just reinventing some aspects of the public school system in the private sphere. But that is because parents, rightly or wrongly, feel they have zero influence over how the public school works, so they just sideload their own version.

(I would say that the parents are right about having zero influence, as quite a lot of American public schools are so big and so bureaucratized that parents do not have a real voice without herculean effort.)

I think it isn’t that unusual for homeschooling parents to form groups, you can do an art class together (otherwise hard to afford), start up some recurring social events, that sort of thing.

K-12 school is sort of a weird social situation, right? You are mandated to be there (you can’t even quit or find a new job), your manager has the right/responsibility of in loco parentis, your co-workers can’t be fired and their only punishment for goofing off is that they might get nagged a bit, and your worst peers don’t care about that at all. I don’t think it is obviously good practice of grown up social skills. You can see the maladaptive behavior that sticks around after—office gossips, bullying, that sort of thing (I mean, that sort of behavior is present everywhere, but I’m pretty sure it is enhanced by the fact that these are strategies to win in the pressure cooker).

You get better at what you practice.

If you practice unproductive social interactions and unhealthy coping skills all day, you will get better at unproductive social interactions and unhealthy coping skills.

> How does staying at home with just your sibs fair better?

This is a very reductionist view of homeschooling.

While some folks certainly do have this experience when homeschooled, a well-designed home schooling experience will have an abundance of social interactions with non-family members.

Sports is an obvious one, but there are also many homeschool groups that engage in learning activities together.

> We all get better at a talent by practicing it.

Exactly. Which is why kids need to practice their social skills in environments that actually reflect how real-world societies functions, rather than being sequestered in an institution with utterly distorted, artificial social structure.