Repost bots discourage new users and kill engagement.

Suppose you're into trampolines but there's no community for that here. You create one, and to kick start it you get a repost bot to repost from the other place.

Over the coming weeks a few fediverse users see your community but see that they're all reposts with no comments, there's nothing to really engage with there, so no one does.

Next month someone signs up to the fediverse and joins a few special interest communities. /c/sports, /c/gymnastics, and /c/trampolines. There might actually be real posts and discussions in /c/sports, but that new users feed is overrun with reposts in /c/trampolines. It's a very clear signal that there's no content here, all the content is on reddit.

If you actually want a community, then you can read about trampolines on the other site, and manually repost things yourself as a human.