We all have friends like that, who promise, and fantasize, and disappoint, then somehow contrive to charm us all over again. In the history of Western art, Leonardo da Vinci is the supreme example of this personality-type, the great non-deliverer.
It’s interesting to speculate why his legend remains so powerful. Perhaps, most of all, it is because of his restless curiosity about so many aspects of nature, recorded in notebooks that place him well in advance of his time - so far in advance that his discoveries, and the conclusions he drew from them, had no immediate consequences.
Then, too, there are his mechanical inventions, which would never have functioned in reality because the technology of his time was incapable of realizing them. They are predictive, but not practical.