I can visit every single style of martial art in 300miles of my location and I guaranty I wont find one that does palm strikes to the head instead of punching as regular training from day one. Most every place I have ever seen or trained at It was something supplemental, and only occasionally practiced.
Just throwing in my two cents here. Ok, maybe only my ha'penny!
Ahem. Please read the following in a light, friendly, conversation tone, and envision, as you do, a rather silly looking, but well-meaning smile.
The very first self-defense technique my school teaches to new students is a palm strike to the face. Followed by three drills utilizing body and side of the neck punches, then an elbow to the floating ribs, a knife-hand strike, same location, then a variety of kicks to the body. Not until the 52nd drill is there a closed fist to the head taught, and then only a finishing technique after a takedown, when the attacker will, (hopefully and hypothetically!), present a simpler target. This technique, part of the fifth set of ten, is not taught until Probationary Black Belt, so students should have a minimum of 6-8 years of training before we really start advocating closed fists to the head. After this first one, there are maybe, maybe, 4 or five more, in all 48 techniques that follow.
I don't know where you hail from, perhaps the martial arts situation is different, but, out of the schools that my schools encounters on any sort of regular basis, I can't say that head-range knuckle strikes are exactly prevalent in gear-free sparring or one-step/self-defense techniques. The Tai Shing Monkey Kung Fu school loooooves to push strikes right through your face, but I can't say I've ever seen them come at me with a closed fist, it's always a palm heel, or a forearm, or an elbow, or even a shoulder. There's a Karate school that fights quite similarly. When we play, there are punches galore, but they tend to be body, not head. Another karate school we see a lot of, but have never sparred with has self-defense one-step type things that, as far as I have seen/been shown or taught, tend towards groin kicks, elbows, takedowns, body shots, and open hand techniques to the face, ears, head...
To be sure, they are karate schools. Yes, I'm sure when they perform Taikyoko Nidan, they do at least 6 head strikes in the form. I'd also note that that is the SECOND form taught, the first has, generally, body punches, as does the fourth. If you go through the traditional Karate forms as I know them, there are a good deal more middle punches than high punches, in the taikyokus, the pinans, the nahainchis, Bassai Sho and Dai, Jion, Tencho, Sanchin.
Leaving out blocks, you know what the strikes are, in order, in the very first form from the Kung Fu school I mentioned? Two palm strikes to the face, a palm to the face, elbow and a low round kick, palm to the face, elbow and a low round, palm to the face, elbow, low round, palm against the back of the head, driving to the ground, then two hammer strikes to the ribs, hammer low, hammer high, hammer low, hammer high, side kick to the body. Not a single knuckle strike to the head. In the next two forms, as taught to me, there are only two closed hand strike to the head, both backfists which come from only a few inches away to the nose, not exactly knuckle breaking force, and a multitude of palm strikes, rakes, and elbows to the head.
I just began training at a Shaolin Kempo school a couple weeks ago. The punches to the head may be coming, but so far, their SD one step type stuff, they call it kempos and combinations, has consisted of upward throat punches, body punches, body and knee kicks, and take downs.
Perhaps in your area it is different, but it may be a TOUCH of hyperbole to claim that all schools within 300 miles teach head punches as the standard until the advanced levels. In my area, the opposite seems to be true. Most martial artists that I know seem to acknowledge that, while a head punch can be devastating to the target, it requires a good deal of training and conditioning, and even then, is a risky proposition that should NOT be your go to move.
If a school is point-fight tournament oriented, then I'm sure they spend a good deal of time connecting their padded knuckles with their padded foreheads, but that's really a different animal.
Just to be really pedantic, there are likely several hundred martial arts schools within 300 miles of you, no matter where you are. I find it unlikely that, say, the tai chi chuan schools are teaching face punches on day one. The Baguazhang schools? Probably not.
I agree that there is a harder, more destructive impact with knuckles than with palms. Simple physics decrees it must be so. The fleshy palm can be more compressed, meaning that the force is applied slower, and thus is able to spread out across both surfaces, resulting in significantly lower PSI.
The problem is that that lower or higher PSI is transmitted 50/50, half to the head, half to the knuckles. You want to know what your doing before you take a full force blow to your knuckles, and, at least with the schools I am most familiar with, in New England, that fact seems to be recognized clearly in training. The One school I can think of that loves knuckles to the head is a Shotokan school, and they train with some very old and grungy looking, really hard makiwaras. They also are fans of placing makiwara boards on trees, instead of on the flexible post. They punch to the head but, they acknowledge that you want to be reaaaaally ready for it.
In short, punches hit with a higher PSI, given equal force of attack. Palm strikes are padded, thus hit with a lower PSI. What is worse for a head, is worse for your hand, and what is better for your hand is better for a head. Also, I bet you have a few schools within 30 miles, let alone 300 that recognize this, and that they are more focused on self-defense, and less on tournament point-fighting and forms.
I could be wrong, but I am recklessly applying my own experience to you. (I know, bad science, that, but I'm just that lazy!)
And remember, my tone is light and conversational, accompanied by a pleasant, if rather goofy, smile.