system
Given the following post, respond as if you're commenting on Reddit:
human
[Dune] Why does House Corrino give Arrakis out as a fief instead of keeping it in as part of a Royal Demense?

It seems weird to me that they would allow such a source of wealth and power to be controlled by one of the other great houses. Surely nobody would be able to object to what is literally the most important planet in the known universe being under the direct administration of the Emperor (and even if they did they’d have to deal with the Sardaukar *and* the Emperor controlling the Spacer guild with their monopoly on spice).
gpt
The emperor is still a player in the Landsraad, consolidating too much power would paint a target on him. He can stand up to any individual house, or even several houses but if enough people get sick of his shit they will gang up on him. 

Then add into it the guild probably wouldnt be too excited about the power consolidation here which leads back to pissing off the wrong people. 

Finally look at the day to day logistics of arrakis, the place is a god damned nightmare to rule over. Its enough of a carrot for the other houses, as in "Hey look over this very important piece of land and you will become more powerful and wealthy than staying on your backwater planet. No need to thank me for giving your house this opportunity, just remember that the emperor is entrusting you with this fantastic boon on your house". From there you can use it as a threat or a reward in back room dealings. "Hey take care of this problem and Ill reward you with Arrakis" "Hey if you dont deal with this Ill take away arrakis from you and doom your house 

Then the other houses will try to curry favor to be the ones put in charge, which means less people making moves against your leadership. Which means they fight each other trying to get into your good graces which means fewer assassins which means prolonged leadership.