The reason knowing the theory is so important in the Sicilian is its sheer popularity.  It has been one of the favorite defenses to 1 e4 since at least the 1950s at even the highest levels.  The top grandmasters of every generation have worked out many of the main lines to move 25 and beyond.

If you are playing a Sicilian from either side against someone who is more familiar with the theory, even though they may be an objectively weaker player than you, they may exploit your errors to gain a winning position known to theory - without having to actually "play" you at all, by just remembering what the GMs do.

So often in the Sicilian, too, the correct move is counter-intuitive:  the "simple" developing move may lose by force because it squanders a tempo you need for what you REALLY should be doing, or fails to block the opponent's known plan.  And unless you follow the current GM scene closely, you might miss a game that changes the evaluation of a line completely overnight. 

Any sharp opening is subject to such rapid change, too, of course, but the Sicilian's huge popularity (check any large database for distribution of games by opening) makes it more likely your opponent will have specific knowledge of lines which run very deep than in some other.

If you want to play it - and it is the greatest counterattacking defense in chess - LEARN IT.  Practice in club games and in casual internet contests, and play a good number in all the major lines until you feel comfortable in the various positions. 

IMO, the reason for the current popularity of what we used to consider "irregular" lines for White (like the Closed, 3 c3, 3 Bb5, Gran Prix, etc.) is that White players often don't want to spend the amount of time needed to learn and keep current with the main lines of the Sicilian, but want to continue playing 1 e4.  White can still pose some opening problems for Black with any of them by virtue of his move in hand - but nowhere near the critical situations which arise with regularity in many of the traditional main lines with 3 d4.

Yeah - extreme sharpness (in some lines - not all of them) combined with very rich positions (I mean, they're extremely sharp, have been worked on for decades and still they're not worked out entirely) combined with popularity.

But don't forget that this holds only for some lines - Bg5 and Be3 Najdorfs, Yugoslav attack Dragons, that sort of thing. If black plays the Accelerated Dragon, theory is much less important. Similarly, white could play quiet Be2 or g3 systems, which are still dangerous for black, but need far less theory for the white player.

So in that light, the popularity of the antis (that give black easy equality) is a bit odd. On the other hand, currently I play 2.c3 myself, so who am I to talk :-)

play the sicilian with someone weaker than yourself or play the french with someone equal or stronger