The problem with that is, can you really trust most instances out there? If you’re a sketchy admin, it’s not that hard to convince a handful of people to use your instance and have a couple dozen anonymous votes at your disposal to influence certain topics. There’s no way to detect it, not even the other users.
That would then mean that small instances would have to prove themselves before being accepted in the wider network of instances and just end up centralizing the fediverse.
With the votes being public, while you can create as many accounts as you want, you still have to publicly use a bunch of bot accounts which makes it more easily detectable. And of course, there’s no way your instance can get away with impersonating you, because you could see it sneaking votes or comments.
I wish it could be more private, but I can’t think of a way you can prevent vote manipulation without revealing who actually voted for what or rely on trust. Another way to look at it would be, what if Lemmy didn’t use instances but instead some sort of decentralized system where each user is its own entity. How would we obfuscate the votes then? Anyone can publish a message to the network, so you need to tie it to some identity, and you circle right back to the problem.
For privacy, there’s always alt accounts and recycling accounts often. Or treat the votes as if you were commenting “+1” or “-1”.
Unless someone comes up with some crypto scheme to somehow anonymously prove that a user has voted, and has voted only once, and the user has credible history being a real person.
Personally, it’s a tradeoff I chose as the price of entry for being able to participate in this while being fully independent of some benevolent person/organization/company/private equity firm. Nobody can take away my API or my apps or shove me ads. I can post entire 4K HDR clips if I want. I can have an offline copy of it if I want to read on a plane trip. I can index Lemmy, I can search Lemmy.