Just look at the highschool pitchers taken first in every draft- the overwhelming majority are busts- since 2000, 2006 is the only big success
Dylan Bundy might be the best pitching prospect in all of baseball.
Karsten Whitson and Dylan Covery didn't sign in 2010 and the next highest drafted HS arm (Mike Foltynewicz) is 10-2 with a 2.59 ERA this year.
Hobgood has been ruined by injuries but Zack Wheeler (taken one pick after him in '09) looks promising.
Ethan Martin is still a good prospect and Gerrite Cole was the 2nd HS arm taken in '08.
Jarrod Parker & Bumgarner were the first two HS arms in '07.
And like you mentioned, Kershaw was in '06.
How is that an overwhelming majority of busts?
Regardless, none of those prospects were considered as good as Giolito (pre-arm injury) coming out of HS.