As well as the knuckles, the hand also conatins one of the hardest parts of the human body with which the stirke. It is simply not necessary to dedicate large amounts of time conditioning the soft parts to become hard, in many cases permanently deforming yourself in the process.
It is, if you want to be able to use the spearhand strike. Which is, after all, exactly what is being discussed.
Now, I'll agree that there's no reason for the vast majority of people to condition the fingers and hands to this extent.
But for those few who do decide to use this strike, the conditioning is not nearly as horrible as you make it out to be. Assuming it's done properly, of course.
I don't train the spearhand now, but I did as a younger man. And my hands work just fine. Be sort of impossible to do my job if they didn't.