Earlier quoted context omitted.
Fundamentally, because we are still worth something even when our achievements are completely removed. If you take the view that our achievements are necessarily integral to our self worth this logically isn't true.
> we are still worth something even when our achievements are completely removed. You can’t logically derive this from anything. It’s just a choice of how to see the world. “Worth” is inherently a subjective value judgement.
Most of what we do and how we think is not “logically derived” and is extremely subjective; we’re human, not Vulcan.