I'd argue it's because nobody submits legitimate Cassus Belli enemy more. It's a song you've heard from me before, and I think it's an important aspect that players, particularly rulers, should focus on more both when declaring war and changing diplomatic status's.
Ask the man on the street in Perdan why his realm marches to war against Xavax...
"We must defend Alara!"
"Why"
"Because Xavax is Imperialistic"
"So?"
"And the Alaran's can't defend themselves"
"So?"
"..."
"Have you actually met an Alaran?"
"...No"
"Really? No family? Friends? Special business interests?"
"No..."
"So why defend someone you don't know?"
"I-"
"Do you have anything to gain? Is Alara paying you?"
"No"
"Did Xavax insult you? Declare War on you? Invade you"
"No they did not"
"Have you ever actually met a Xavax?"
"No, well, except for some refugee nobles from their civil war"
"And do they have any claims on Xavax lands or titles?"
"No"
"So why are you invading Xavax and killing its peope"
"I dunno, war's better than boredom I guess."
