There may be disadvantage, yes. But it could also be that pain asymbolia is fine in a creature as high-level as a human—but without any selective fitness advantage, what would drive it to fixation in a selective sweep? Given zero reproductive advantage and possible disadvantage, it’s no surprise that it’s rare.
Probably because the negative feelings about the pain are what strongly motivate you to avoid it, and hence avoid physical damage.
There may be disadvantage, yes. But it could also be that pain asymbolia is fine in a creature as high-level as a human—but without any selective fitness advantage, what would drive it to fixation in a selective sweep? Given zero reproductive advantage and possible disadvantage, it’s no surprise that it’s rare.