For the same reason you presumably didn’t turn it up in Prey. You didn’t want to do the designer’s job for them. So why turn down the difficulty so that the dragon will have less hit points? Isn’t that tinkering with the designer’s job? Instead, to NOT do the designers job for them, when a game gets hard or frustrating, you don’t tinker with the difficulty and keep playing. Just like you seem unwilling to raise the difficulty when a game is not challenging enough, you should be unwilling to lower it for the same reason, right?