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.

Lemmy is basically just a decentralized clone of reddit. A public profile containing comments, posts, upvotes, and the like are considered an integral part of the Reddit experience. I personally don’t like it as a feature, but that’s why it exists.

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.

Mastadon, Searx, Fediverse, and so on aren’t killing or replacing the sites they’re modeled after, not even close. They’re just providing a privacy focused alternative for those who don’t want to whored out by corporations or abused by powermods or shitty business decisions