As one of the other replies (nested too deep to reply to directly) said, many of us were raised to be humble and self-effacing, especially about skills related to innate abilities like intelligence. So it feels unseemly to say, in essence, "Hey, you should hire me because I'm great at X, Y, and Z." It feels weird enough to list skills and accomplishments in a resume, but overtly selling yourself feels wrong.
Maybe people like us should team up in pairs and promote each other. I'd have no problem talking up a colleague I knew to be talented, far more forcefully than I'd ever do for myself.
that only works if we know each other very well. every time someone tried to talk me up i felt more awkward than if i had done it myself, because that person didn't know me well enough to actually judge that. the only talking up by someone else that i can tolerate is: "i have worked with this guy and i would hire him (again)"
Maybe people like us should team up in pairs and promote each other. I'd have no problem talking up a colleague I knew to be talented, far more forcefully than I'd ever do for myself.