conditioning

[Kuma]

You can get callouses on your knuckles from repeated punches to a brick wall, a hard surface, etc., but you do not need callouses on your knuckles to be proficient at punching.

Ouch! You never want to repeatedly strike a wall or other solid object like that. The makiwara gives when you hit it so it absorbs the force you put into it. Hitting a wall, however, just sends all that secondary reaction force right back into you. Not good at all.

As said before, calluses are a side effect, not the main emphasis, for hitting a makiwara. Makiwara training primarily teaches you focus and power for your techniques.