There are two reasons, basically, why soccer lends itself to spectatorial boredom. One is that the game is mercilessly hard to play at a high level. (You know, what with the whole “maneuver a small ball via precisely coordinated spontaneous group movement with 10 other people on a huge field while 11 guys try to knock it away from you, and oh, by the way, you can’t use your arms and hands” element.) The other is that the gameplay almost never stops — it’s a near-continuous flow for 45-plus minutes at a stretch, with only very occasional resets. Combine those two factors and you have a game that’s uniquely adapted for long periods of play where, say, the first team’s winger goes airborne to bring down a goal kick, but he jumps a little too soon, so the ball kind of kachunks off one side of his face, then the second team’s fullback gets control of it, and he sees his attacking midfielder lurking unmarked in the center of the pitch, so he kludges the ball 20 yards upfield, but by the time it gets there the first team’s holding midfielder has already closed him down and gone in for a rough tackle, and while the first team’s attacking midfielder is rolling around on the ground the second team’s right back runs onto the loose ball, only he’s being harassed by two defenders, so he tries to knock it ahead and slip through them, but one of them gets a foot to it, so the ball sproings up in the air … etc., etc., etc. Both teams have carefully worked-out tactical plans that influence everything they’re trying to do. But the gameplay is so relentless that it can’t help but go through these periodic bouts of semi-decomposition.
But — and here’s the obvious answer to the “Why are we doing this?” question — those same two qualities, difficulty and fluidity, also mean that soccer is uniquely adapted to produce moments of awesome visual beauty.