I've always wondered about that though.
I think the brutal truth is that most instructors/practitioners simply don't know most of the kata grappling applications themselves. It's more of a trial and error approach with mixed results and stumbling into things.
So I kind of question how much standing grappling is actually found in kata or Karate because if we are being brutally honest no-one can say without reasonable doubt - that this x or y application is a grappling technique.
Outside the few verified grappling techniques - the rest is just pure speculation/assumption. It's like picking up and trying to fit one puzzle piece at a time - until it fits into the entirety of the puzzle. The difference being we don't even know what the entire puzzle looks like - we can only discern what it may look like.
Then there is the even more concerning point that you rarely see anyone in the Karate world apply these grappling techniques (found in kata - outside of the very basic ones) in actual kumite or sparring. If you can't apply something in sparring - it's a theory in my eyes.
I know exactly what you mean and these doubts are understandable. Uncovering historical (or "true") Karate bunkai is somewhat archaeological - you dig up old texts and compare structures of kata in various styles, trying to connect these "old bones" into the skeleton of some "common ancestor".
Luckily, we have guys like Patrick McCarthy and Iain Abernethy who do this kind of thing and then successfully re-create many of this "common ancestor's" techniques. The foundation is of course The Bubishi - the so-called "Bible of Karate", a Kung Fu manual studied and revered by most (if not all) of the original Karate masters. Application of Karate kata can often be traced directly to techniques of The Bubishi - and that's a start!
https://iainabernethy.co.uk/article/bubishi-–-karate’s-most-important-text
As for application in sparring - well, obviously with WKF rules this is impossible. But I am optimistic. I do believe that a new martial arts era is upon us. The popularity of MMA has put a lot of pressure on TMAs for MMA-like rules, MMA-like practices, MMA-like applications. Karate doesn't have to borrow these from MMA - it already has them in the Kata, they just need to be pulled out of obscurity and back on the mat.
PS. Just yesterday I passed a new Aikido dojo called "Ultimate Aiki", marketing itself as "Aikido with elements of MMA". See, the revolution has already begun!
You're absolutely right that most instructors/practitioners don't know, but some do--they may be scattered across different styles, in various countries, but they are out there. We have a number of legitimate instructors teaching material handed down to them through their lineages who include the standing grappling components of kata; Oyata, Odo, Toma, Iha, Maeshiro, Yamashiro, Uehara, Higa, Motobu, and more, just to name a few Okinawans who've been masters in the last 50 years. Do they all teach these things in completely practical ways? No, not always--Japanese formality has snuck into a lot of cultural practices on Okinawa--but you still have a starting point. Add to that the cross-training experiences that they and their students incorporate, and you can find a good bit of solid material, even before you have to start the trial-and-error process, so you should have a reasonable base to work from.
Of course, you're absolutely right about the lack of resistance training. I've known a good number of karateka who will attend bunkai seminars and memorize a bunch of applications to a kata, but never really properly TRAIN them. Sure, they'll perform them by rote against whatever basic attack was initially prescribed, but they don't go beyond that. My Sensei was big on pressure testing and sparring with everything, so that was a mentality he fostered in me, as well, which is why I try to have students do so many different types of sparring. The grappling components need to be focused on with sparring specifically for them to really develop a good working understanding of how to use them, of course, and then you can integrate it into other types of sparring.
I do think that more and more people are moving to the more practical approach for karate, even if it's still a tiny fraction of the overall karate community. My hope is that it will continue to grow, but unfortunately I think there really needs to be a solid competitive aspect to really foster that, and we really don't have one, yet. Karate competitions are a joke, and even the full-contact ones don't incorporate any of the kata methods, really. Kudo/Daido Juku is full-contact and allows grappling, but it's really just Kyokushin and Judo mixed together, rather than anything to do with the kata methods. MMA is best option for open-ruleset pressure testing that we currently have, but almost nobody joins a karate dojo with aspirations of becoming an MMA fighter, and the idea just isn't fostered by the vast majority of karateka and karate instructors. I've been working on a competition kakedameshi format, because while it is a very limited form of sparring, it is a good way to spar with the standing grappling methods of karate, and if we could get a competition circuit going for it, more karateka would likely participate, which would improve overall awareness of those methods, as well as the overall skill in using them within karate, as a whole. Getting something like that off the ground is tough, though
