The Guico Piano arises from 1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 - 4.c3 isn't mandatory to make the opening a GP. 

Consider the following stats from ChessBase's 2012 BIG DB (5.5 million games)

3.Bc4         55,000 games         54% winning percentage (W)

3.Bb5 a6   167,000 games         56% (W)   

3.Bb5 ...?  70,000 games         +56% in all lines except 2 (3...f5, 3...g6)

The Game Explorer (Master Games) shows

The Ruy Lopez is about 3x more popular in the CB BIG DB and 5x more popular in the Game Explorer...

those are some of my better games of the sicilian

There are 2 reasons.The first is that, e4-e5 openings are not played so often.
Secondly,it is not give any advantage for black. For example.

Well, I guess that settles that. 1...e5 is for dimwits and so nobody plays it any more, because the Sicilian kicks ass every time.

not true e5 is great

After 1.e4 the Game Explorer shows e4-e5 openings are in fact played so often

The Giuco Piano isn't inherantly boring, but only seems that way because so many games throughout history started that way and it has been analysed to death.  I was studying a Petrosian game and thought, "Four Knights game?!  Good grief, but I'll stick with this study" Then, things started becoming imbalanced, and even interesting!  For example, Petrosian made an eventual Qh5 move (presumably) meant to prevent black from playing ...h5, which ensured that white would carry on with his pawn storm.  Petrosian was also so obsessed with the e6 square that he forgot an even stronger sac with Nf6, leading to a massive win in material or mate.  On its own the four knights is boring, but that Four Knights' game taught me that the players themselves can make even the dullest openings interesting.  Then again it is the middlegame where the interesting positions are anyway.  

I always hate those lines where white trades the Nc6 then follows up with e5!  Or, plays Qf3, which prevents ...b6 as e5 hits the knight while the queen stares down at the rook.  I mostly like Sicilians with a6,e6, and d6 pawns, usually not moving the b-pawn until some other pieces move first.  Though, I like a bishop on e6 as it watches the usually weak d5 square.  I also usually know when to push for a ...d5 pawn break or even ...e5 if white has an f-pawn out like in the f4 open Sicilians. 

Because Black achieves close to an equal position

Oh, here's the Petrosian game I was referring to:

I used to play GP but I found that it lacks the teeth to force your opponent to be on the defense. Too many opportunities for your opponent to grab the initiative. As a class A player, I tend to play masked openings, meaning it could be started with something that may look unstandard and eventually develop it to a strong book opening.

I play the Italian rather frequently and here in Wisconsin it's undergoing something of a renaissance...

I play it for black. I am undefeated in OTB games with it from both colors. I occasionally play it as white if I need a win. I have two wins as white, two wins with black and one draw as black

I've always liked the Guitar better.

Hm, the Guitar Piano...

Honestly, I was trying really hard to reason why black played that, and in my notes noted it was weak at worst and at best a faulty defensive preparation.  As for the Bxc3 yeah, that was premature and looked like a positional mistake but didn't want to sound like I think I know better than masters, so thought there was at least enough merit behind it (though analysing from the white side wasn't my problem) to be played on Petrosian's level to justify it (somehow).  I still won't take up the four knights anytime soon, but I liked looking over that game and taking note of plans, imbalances, etc.