Because automated reposts just drown out all the posts that people are actually participating in. Yes you could scrape reddit and repost all the content here but lemmy doesn't have the userbase of reddit so your feed would just be full of a bunch of dead bot made posts.
But if a person deliberately reposts something from reddit in a Lemmy community where it fits and adds to the community then that's perfectly fine.
I love you and I'm saving this.