Why does it seem that on larger platforms there are bots that downvote comments?

You can tell it's bots because all the downvotes happen at the same time. I can guess for astroturfing purposes one would want to upvote their targeted comments and downvote all others but on lemmy?

The instance admin can see votes. If you are noticing a pattern maybe ask them to confirm? There can be delays in federation so you might see them all come in at once even if they occurred quite a bit earlier.

I know your question was "why" but maybe confirming that is that is happening vs a different cause can be the first step.

Why, I haven’t been so insulted in my life, being compared to a bot. It’s me.

You look at it as a social media platform. The people who run bot accounts, have specific goals they want to achieve such as swaying public opinion on a specific topic. They could have been paid by a corporation to do damage control on a negative event that happened.

When they downvote you and upvote their other bots, their content gets higher visibility.

Everyone can see votes: https://lemvotes.org/

I don't care about the downvotes. I intentionally view ALL posts in the feed, I don't hide anything just because of low score. The lowest score might be the best!

And your four-thousand clones.

Don't forget about other unnoticed tactic, redirecting your attention to something that has nothing do to with the subject, just to make you disregard entire discussion or pay attention to something different. It's subtle but happening all the time, now you'll see it everywhere.

the epsteins called it narrative management.

I'm now paying attention to this and totally forgot about what OP was asking.

I'm not so sure about that

Unfortunately it doesn't work, returns 404 for everything

It has definitely worked for me in the past, maybe it broke recently :(

I'm convinced its often the admins that the bots belong to.