First of all, high school (for all its positives and negatives--and there are plenty of positives) is NOT the real world. Schools are the only places where there is such a high concentration of like-aged and experienced people. They are, especially in the U.S., carefully fabricated and constructed environments, often with their own sets of rules and regulations different from what one would expect any place else. (You need to raise your hand and ask permission to go to the bathroom? Someone can torment and bully you with impunity and its your own responsibility to learn to "deal" with it?)
Second, homeschoolers interact with many different kinds of people all of the time. Our kids play in community ensembles, participate in non-school sporting leagues, attend music and theater performances (during the day!), and experience the world with us in general. They have close friends their own age and younger and older.
The "home" and the "school" in homeschool is misleading. Many of us spend significant parts of our days out of the home. And to us (and I think to many other readers here) "school" is not synonymous with "learn". A school is a building or an institution. People don't learn because they are "in school", they learn because they want to (or need to, I guess).
We treat our own homeschooled kids as if they are, well, just people. My job as a parent is to provide for them (love, shelter, food, space--I'm an angel investor) and their job is to explore, learn, grow, adapt, think, do.
For us this works better without an institution involved.