Jazz music requires A LOT of theoretical learning and very good technique. Classical music as well, but not as much theory because all of the notes are written on the page.
In jazz you have to be fricken smart because it is all about improvising. In jazz you don't want to sound like "that guy", you want to sound like yourself because anyone can go listen to "that guy". Being "that guy" is a good thing, because copy cats won't get the attention, you will.
Classical requires a lot of technical training because you have to be able to sight read PERFECTLY and be able to play whatever is put in front of you. Usually classical players are hired and put on stage with music that they haven't seem before. Your big wig rock groups spend countless hours rehearsing and working with the musicians that they play with all the time to get that technical music down pat. The classical players can't rehearse like that, so they build the skills to perform just as well without the rehearsal and familiar musicians, and perform their best the first time they see the music.
And if you are a successful jazz player, you can play pretty much anything because you know all of the necessary theory and have the basic skills and abilities to mold your style to almost any genre (middle eastern for example is difficult).
Basically, classical and jazz music requires a ton of brains and skill, making those musicians very versatile in whatever musical situation that they are put into. The big jazz cats definitely deserve their titles and praise.
EDIT: Another advantage that jazz players have is being able to analyze almost any kind of music deeply rather than just passively listening to it for the beat or melody. I enjoy figuring out the chord progressions (stuff like "blitzkrieg bop" using a 1-4-5 progression in pretty much the whole song) and the little nuances that create the sound of that kind of music.
All I can say is, I was classically trained when very young, and I didn't have the discipline to cut it. Although I work very hard to play rock/pop/blues music, I would never lie to you and say it takes tons of discipline. I spend 1/10th or less of the time classical musicians spend training and practicing.
I'm sure "technical metal" is very difficult, but I still have a hard time believing it could be half as hard to play as it is to listen to.

