They’re private in the sense that there isn’t a corporation stealing your data without your knowledge, selling it without your consent, whoring you out for ads against your will, and/or making your experience shittier to manipulate you into buying their paid features. These alternatives offer a much more pure experience for the typical user. Things like comment and vote history being public is just a part of the design of the forum, they’re not tools to farm your data.
They could, but they’re pretty much guaranteed to not be as bad as the big corporations because they lack the resources and know how. The fact that you could also host your own instance is a huge plus considering that you could basically ensure that your data won’t get collected.