You can go outside at night and see big rocks floating. If space is so empty, how is it possible that you can see them with your own eyes?

We live in a patch of space that's not that empty. Maybe that interstellar rock floated from other patch of space that's not that empty all the way over here, all on its own.

Most rocks we see in our patch of space, as far as we can possibly know, were not intentionally launched.