Well, if white plays the open Sicilian, he has to be prepared for a number of different variations from black (even more than in your post, because black can play 2... Nc6 or 2... e6). By playing a different line (Alapin, Grand Prix, Closed, etc.), white can steer the game into a variation he is familiar with, rather than whatever variation of the open Sicilian black chooses.
Why is it that no one ever plays the open sicilian?

Biarien beat me to it. If white plays e4, he has to either try to learn all the possible Open Sicilian lines (Dragon, Najdorf, etc) to be prepared for black's "pet line", or he can just learn one anti-Sicilian (Grand Prix, Alapin, Smith-Morra Gambit, etc) and make black play into his "pet line".
Just like anyone playing e4 as white needs to be ready for the Sicilian, anyone playing the Sicilian as black needs to be ready for all the anti-Sicilians. And this is why amateur players shouldn't bother with the Sicilian - there are more important areas than openings to focus on.
--Fromper

for me there isn't problem, as black i find to be comfortable with the anti sicilians too(most with the alapin in wich I plai the d5 line)

It depends what level you are at to some degree, for the super GMs an open Sicilian is the main way to go if you want to win, although they sometimes play the others. At other levels it depends on where you are, whether you like studying tricky lines whilst trying to keep up with the latest novelties. And a quick glance at james1996's live games shows he has lost with a lot of closed Sicilians :-p I think that is simply good on your opponents, they could have played it open and ran into your book knowledge but they instead outplayed you in lesser lines. If the lesser lines are the easiest to understand then why have you been losing....
Not only do you have to deal with a huge variety of black systems, but you don't even get anything special for your efforts. Sicilian is considered the theoretically best defense to the king pawn for a reason, so there is even less reason than usual for players at our level to play positions we don't like in order to get the maximum theoretical edge. Run your games through a program and watch how the assessment changes. Do the same with some GM games. Big difference.
So the only real reason to play it is if you happen to like the positions that arise from it, but I personally found that black's positional trumps left me a bit desperate to keep an initiative going, and we all know how that often turns out. Sicilian counterattack for the win.
There is no need to play the most critical variations at our level unless you happen to love the positions it leads to, which is a good thing for the sake of variety IMO.

I actually see the open sicilian as black more than all the anti-sicilians combined, both in online live chess and correspondence. I do, however, see more anti-sicilians over the board but I live in a small town where there aren't many chess players and pretty much everyone at my club except me doesn't study theory, so I guess for them trading a center pawn for a flank pawn in principle doesn't make a lot of sense. I play the open as white myself, not just because it is the best but also because I find the positions quite interesting. Unfortunately, I have to review and play certain variations many times before I have a really good understanding of them. The open surely isn't for those who want to cut down on their opening study, which may explain why amateurs often prefer something different.

I think most people are way too worried about their opponents "opening preparation". I play the open sicilian as white and also play the sicilian as black. Since most people don't play the open sicilian I've gotten a lot of experience playing against them and much less playing a main-line open sicilian, and hence as black I'm much more confident when white plays a side-line.
As white when I play an open sicilian and just wing it I do just fine. I don't play the sharpest variations, but I'm still able to get to a playable position black may studied but has very little experience against so he still can't play it well.
So that was a little long, essentially people are way too worried about opening thoery.

In my long gone e4 days I used to play 2 f4 against the Sicilian.
I had a huge plus score with it, never found a good reason to oblige black with an Open Sicilian

Its simple really for the white player that specialises in one of the anti-sicilian lines : its a LOT less work involved ! To play the open sicilian as white requires too much work. There is tons of theory on the Dragon,Najdorf,Sveshnikov,Schevenigen, Paulsen, Taimanov,Kan ....... its a nightmare for white and the black player ONLY has to know his chosen line !

NMReb - your advice sounds exactly right. And: could you please recommend which line a 1600ish Black player would find most fruitful to study and play:-))

I play the sicilian (open- nardj) for dynamic play, since it is whites "option" to comply with my desire (not!) leaves me with but one sure option against (e4) because the slow but steady progress playing the closed sicilian is not for me (e4) gets the (alekhine)! win or lose we "will play dynamic chess"!

NMReb - your advice sounds exactly right. And: could you please recommend which line a 1600ish Black player would find most fruitful to study and play:-))
As black you will have to learn something for all the anti-sicilian lines and there are books that cover them. As for a good sicilian as black against the open lines there are several : najdorf, dragon, sveshnikov, schevenigen etc. You should check them all until you find something that you like/prefer. I prefer the sveshnikov and classical sicilians as black myself and play them both. I have also played the black side of the najdorf, and dragon as well but not often.
Those of you who are sicilian fans and play it "regularly", you know what I mean. Players nowadays only play the "grand prix attack," expecting to sacrifice a queen to give checkmate with two knights. If you ever acheived checkmate with two knights using the, "grand prix" attack , please speak up. Here's the open Sicilian:
I know there are other variations in the Sicilian, but these are the easiest to understand. This, is the true nature of the sicilian. Believe me!