Deprivation is like a world with certain colors missing. Chronic underearning or anorectic spending are adaptive responses to living in a world that feels too damn scary. These are subconscious devices that struggle to reconcile the outer world with an inner world of fears and doubts about ourselves and our abilities. In some ways, under earning is a protective barrier against rejection, competition, and envy. Underearners do not do well with these. Neither do they do well advocating for themselves.
It is not merely a bit of bad luck when a woman with two Ph. D's cannot, for the life of her, find a way to earn a living wage. Similarly, it is not from a lack of clients willing to pay a competitive fee that a freelance engineer has never been able to earn anything like what his experience and talent would realistically have been expected to bring.
Underearning is not about low income. It is about low self-esteem.
There is a world of difference between someone not earning very much right now, or whose abilities are limited, and the chronic underearner. Underearners don't underearn because the world does not pay them enough. They underearn because they subconsciously find a way for the world not to pay them enough. And, unsurprisingly, I guess, have a way of sabotaging things too. Even when they do muster the courage to get out there, to assert themselves, they always seem to be driving with the brakes on.
Underearners find a way to shut down their desires. The problem is, when you shut down certain desires you always overshoot the mark. You don't wind up shutting down only the things you aimed at, all of a sudden you find the shutdown spreading to your desire for food, sex, or intimacy. Even the very appetite for living a vital and involved life can go by the wayside.
Many of the underearners I have known found a way of getting very cozy in their caves. Deprivation isn't hard, they're used to that. It is going after abundance that scares the hell out of them. Even when financial abundance does happen their way, they tend to feel distinctly uncomfortable about it. If a sudden windfall comes along, especially an inheritance, it can even precipitate a fall into depression.
The idea of having a sense of plenty, security, and abundance, and then losing it, is far scarier to an under earner than never having had it in the first place. You can imagine, then, finding a way to protect yourself from that, even if it seems counterintuitive to the rest of us, seems perfectly reasonable to them (at least subconsciously).