I'm assuming that a belief in reincarnation is implicit here.
Karma entails recursion, which entails determinism. If I am evil in this life, then I will be born into poverty in the next. If I am virtuous in that life, I shall be born into prosperity in the next. If I am evil in that life, the sequence repeats monotonically. But it goes either way. If I act viciously as someone born in poverty, then my next life will also reflect that. If I'm born with a disability, is it my fault if I become misanthropic as a result? Well, one would argue that it's a consequence of your past life, and hence you must act virtuously in this life in order to be given a better shot, or something like that. It's more complicated then that, but the causal difficulties essentially boil down to this.
I could only be born into prosperity by breaking the chain. If at any point in the sequence I am consistently good, I may be birthed as a Buddhist or Hindu (which hold priority over other religions, if reincarnation is true). And if I devote myself thereafter, I may transcend the cycle of rebirth. But if I do not, then I am at risk of sliding back into recursion.
This requires a metric to weigh good and evil. We don't have this, but maybe our souls have weight, or something. In any case, we're like bystanders in a hypothetical cycle of rebirth. We'd prefer if someone else took the initiative rather than ourselves. It's probably easier to assume that karma does not exist. The implications are easier to bear, yet the incentive to act virtuously is negated. If you want to prepare, then it's more rational to show kindness.
But then again, we are superrational beings. Who knows if our kindness is genuine?