The promotions confirm it. The salary confirms it. You're not unaware of the evidence. But there's a gap between knowing you're competent and actually feeling it, and that gap tends to fill with constant proof-seeking: another win, another round of validation that settles things for a day or two before the doubt comes back.
For a lot of high performers, this pattern was wired early. Achievement became the way to earn approval or safety, and a conditional sense of self-worth got established long before the career started. Because the next result always comes and always proves insufficient, the doubt doesn't resolve. It just gets more expensive to manage over time.