Because in general, making things more difficult for an adverse party has great potential for also having side effects for proper users.
And conversely, making things easier for proper users very often also makes it easier for adversaries.
You’re right that it’s not a strict 100% rule, but it’s so common a pattern that keeping it in mind when making security decisions (to avoid fulfilling it or at least weigh risks) is a good practice.
Because in general, making things more difficult for an adverse party has great potential for also having side effects for proper users. And conversely, making things easier for proper users very often also makes it easier for adversaries.
You’re right that it’s not a strict 100% rule, but it’s so common a pattern that keeping it in mind when making security decisions (to avoid fulfilling it or at least weigh risks) is a good practice.