Ueshiba: 1915-1919 and he started teaching. Was being regarded as strong. He only got better. Never any long time frames of training with Takeda.
Tomiki: 1925-1936 and he started teaching ... in Manchuria.
Shioda: 1932-1937 and then sent to China. Supposedly trained on and off for about ten years.
Tohei: 1940 - Noted as being only 6 months before teaching.
Why did it take them so few years to become good? Tomiki had a background before he met Ueshiba and it did him no good. Ueshiba had a background when he met Takeda and it did him no good. If those backgrounds were so solid that they "helped" them get better, why was it that every one of them (Ueshiba meeting Takeda, Tomiki meeting Ueshiba, well, everyone meeting Takeda and everyone meeting Ueshiba) was tossed like a rag doll and treated as if they were children in the hands of a parent? Their prior training counted for nothing. Their prior training could do nothing to stop or counter anything.
So, Ueshiba, with all his prior "training" gets manhandled. But then goes on in less than ten years to become someone who manhandles. Tomiki with all his prior training gets tossed about effortlessly like a rag doll some 63 different ways but then in very little time starts tossing judoka around. Tohei brags that it only took him 6 months (but not under Ueshiba's teaching).
You missed Tenryu, who did all kinds of pushing with Ueshiba. In three months Ueshiba told him he "got it" now no one could throw him. What did he show him that Tenryu could combine with his training?
You missed the Judo guy who was unthrowable in matches
Funny how if you read and dig, the ones who got it, and were seen here and there doing it -all demonstrated tricks to demonstrate it outside of waza.
As to why?
I'd bet because they were taught the body skills to make aiki
Everything else is icing on the cake.
If you recall the Judo guy who gave a large donation was given a "gift" in return. they...brought him to an aikijujutsu guy who showed him what? Waza? No. he demonstrated aiki power through static tricks to let the American see real power, that he was told made certain guys unthrowable. Further, that few men knew of it, and fewer still practiced it.
I think it is obvious that it is rare and it not openly taught. Thousands sweat it out and spend thousands looking for it. For some smart guys -technique 267 variation b. didn't come close to cutting it, they started looking elsewhere.
Not to worry though, Sagawa, Kodo, Hisa, Ueshiba, all had many contemporaries training with them who never got it.