This is a common Western misunderstanding of karma.

Karma isn't about guaranteeing consequences/rewards. It's about actions increasing the odds of one or the other.

While it certainly has been used as a part of cosmological frameworks involving reincarnation, it can also be used in a secular framework within the context of a single life without relying on notions of a metaphysical force.

https://en.wikipedia.org/wiki/Karma