Hoagies Gifted Education Page has a webpage dedicated to underachievement, including resources discussing research, and how to reverse underachievement.

Not knowing the OP's childhood experiences beyond what was mentioned in this discussion thread, I'll just say in general:
Some kids may not experience an academic/intellectual challenge in school, and may be missing the joy that comes from persistence in the face of difficulty until achieving success. Kids without a challenge in school may often experience being treated in a negative manner as compared with their classmates:
- needs may be overlooked/ignored,
- expected to "tread water" until other students catch up,
- reprimanded for not paying attention,
- expected to teach other students rather than learn something new.

In extreme cases, a child who is "ahead" may be publicly shamed in the classroom as a means to get them to slow down and "fit in"... allowing others to to make relatively greater educational gains toward closing the achievement gap and/or excellence gap.

Unfortunately, by a practice of cutting down the tall poppies, children may be unintentionally or deliberately taught to under-achieve.

For some of us, this is why we:
- continue to advocate,
- support the use of the IAS for acceleration,
- believe in alternative educational paths,
- encourage gifted individuals (whether children or adults) to reflect on their experience and learn about concepts such as underachievement and imposter syndrome... so they can face the damage which may have been done and begin to heal, embrace challenges, and realize more of their full potential.