Well I don't know why it's being done like this, but my informed guess would be:

Resilience. If the content wouldn't be copied, defederating/blocking an instance would mean that the content you created there (topics, comments, etc) would be lost to you. So if you wrote a nice comment, or saved a bunch of topics for later, and then your instance blocks the other instance... that would be gone for you. With the copy this doesn't happen.

Performance. Instead of having to deal with every user (from a different instance) individually, your instance only has to deal with other instances. With this updates between each other can be sent in larger chunks (and definitely with less network connections). Additional benefit: smaller instances don't get knocked down by user-heavy instances when they host a popular community.

Just guesses tho.

All social media is a liability time bomb unfortunately. That's why only the biggest players can afford it so far.