Because technique per se is not that difficult to acquire. Acquiring repertory (memorising pieces) is what really takes time.
Well, this assertion is more than strange and makes one wondering if you mean what you write, you write what you mean, or you know what are you talking about. To memorise say Liszt Sonata, or something of similar difficulty takes about 10-14 days. Mastering to more or less performing standard as well as tackling its technical difficulties takes years. Mind you, this is when you got very solid technical background.
Or do you think somebody like Dinu Lipatti was completely out of his mind when he said that to learn Tchaikowsky 1st Concerto he needs three years? Do you really think he meant "3 years to memorize the piece"?

The memorisation is the least of the work. In fact, the whole work (musical and technical, which BTW, is the same) on the piece just starts after it's been memorized.
By skipping the exercise and using the piece/passage to acquire the technique, not only you will be acquiring technique that does not require ulterior adaptation, as you will be learning memorising the piece/passage straightaway. Now that you are in the splendour of your youth, time may not seem that important.
As nice as it sounds on paper (or looks on a screen

), I do not see any relevance or correlation with reality.
Let me give you a few examples.
Somebody like Richter, Rubinstein, or Neuhaus were going the route you propose and did not get systematic technical rutine as kids and youngsters. As a result:
1) Richter had to spend 12-14 hours of practicing his technique to keep himself in the shape and to become who he is (Mind you, I strongly believe, if he would've gotten that technical foundation as a young kid, he would never have to struggle and had had much more time to learn even bigger repertoire).
2) Rubinstein's technique was quite lousy for the most of his life (except of late in his life, when he finally "found the touch")
3) With all his artistic genious and immense repertoire Neuhaus was just a lousy pianist.
To my knowledge MOST of the great pianist got exhaustive schooling as young kids.
Friend of mine graduated from Kharkov Conservatory, where he was studying with Regina Horowitz. From her words on one of technical exames her brother Vladimir Horowitz presented entire Czerny Op.740 (!!!).
Or do you think somebody like Rachmaninov had no idea what he was doing when before concert tours he'd play for a week only excercises and etudes?
Or do you think somebody like Heifetz was an idiot when if for some reason he did not practice for one single day of his life, the next day for 9 hours he'd play scales only?
Best, M