system
Given the following post, respond as if you're commenting on Reddit:
human
[Dune] Why were the Harkonnen allowed to have thiefdom of two planets, while the Atreides had to give up Caladan to become stewards of Arrakis?

Both were Major Houses, both were getting Arrakis as a siridar fief so they should have a homeworld so to speak as well.
gpt
There are no set of rules how many or how few fiefdoms a house can have, it's all dependent on the Emperor's good will.

And as you might recall the Emperor wanted to destroy House Atreides to remove a potential rival, so he had to make sure that there were no loose ends like an orphaned homeworld.

And really you could justify it rather easily:

A home world for the most important planet in the Universe? 

A small price to pay.