How come there was no mention of this barrier to apology-- a low likelihood of behavior change after the apology?

A "good" apology does not just include awareness of what you did wrong-- it also includes a commitment to never do the wrong thing again. And that part is the toughest. It's relatively easy to learn to swallow your pride and take the hit to your self-esteem; sticking to a promise or a boundary, probably for the rest of your life or that relationship's life, is much harder.

Someone can apologize feeling genuinely sorry for what they have done. But they may still feel they're not up to the task of repairing emotional damage or putting the apology into action. Without this action, they know on some level their apology won't be seen as sincere; and so they may feel it's better not to apologize at all. Better not to be the little boy who cried wolf.