There are a few technical problems with that. First of all, the cost of each instance would become quickly unbearable since everyone has all the duplicated data.
Second problem, a malign entity could just come, create its own instance, spam everything and everyone with ads or whatever and suddenly every instance is full of that stuff. Also, how do you handle defederating in that case?
What has been proposed before instead was to make some kind of mega communities that gather all posts from communities with the same name across instances