I pointed out that there is no need for a seeming disadvantage to be offset by some redeeming advantage.
You could come up with any number of possible reasons if you were looking for a "reason," although it could simply be a side-effect of some other mutation (such as the layout of the pelvis, intestines and abdominal muscles being more efficient for hunting/etc. without the "extra" parts.)
Here is another important inversion that you seem not to have considered, though: do you think that when externalisation first happened, testicles were as sensitive to pain as they are now?
You could come up with any number of possible reasons if you were looking for a "reason," although it could simply be a side-effect of some other mutation (such as the layout of the pelvis, intestines and abdominal muscles being more efficient for hunting/etc. without the "extra" parts.)
Here is another important inversion that you seem not to have considered, though: do you think that when externalisation first happened, testicles were as sensitive to pain as they are now?