Because it can go horribly wrong, and you don´t want that kind of procedure to go horrible wrong, or wrong at all. Especially not since there are natural ways to get the same, or better, results.
I advice against all kinds of surgery that isn´t neccesary, whenever there´s surgery there´s the risk of complications.
Do a search and have a look at various pics of penis surgery gone wrong, not a pretty sight. Besides, after you´ve done the surgery you have to wear a ADS for a pretty lenghty time anyway, and that is probably responsible for much of the gain. And wearing a ADS after surgery hurts like a mofo. And you have to pay for it too, meaning the JES-extender or something in the same pricerange, recommended by your surgeon.
The major gains are in flaccid state btw, not erect.
Seems like a lot of risk taking and hassle when you could do it naturally instead.
Good luck!