In short: organized sports is *tribal* in nature. Always has been, always will be. In that, it's not really much different from any club (including the SCA), or for that matter any religion. The excitement comes in great part from a sense of belonging to something larger. Unsurprisingly, people are more into belonging to something that seems to be successful, which is why there are a lot of fair-weather fans (including to some degree myself) who get a lot more fervent when things are going well.
Yes, marketing departments take advantage of that, but the cause and effect is subtle -- otherwise, it would be much easier to "buy" fans than it is. Marketing's an element; so is success; so is the general spirit of the tribe. That's why Sox fandom managed to sustain such a level of passion through years of suckage. Had nothing to do with success, and really very little to do with marketing; it had everything to do with grass-roots identification with the tribe.
As for the game being boring: I won't deny that baseball isn't the greatest entertainment around, although metahacker is dead-on that being there in person is very different from watching it on TV. Again, that's entirely due to the tribal nature of things -- it's the difference between *watching* a ritual (and let's be clear, a baseball game is every bit a tribal ritual) and *experiencing* it.
And like all sports, and indeed most spectator activities, the better you *know* the activity, the more interesting it becomes, because you are watching the subtler nuances. Heaven knows I disliked football for many years; I've wound up with that being my favorite sport precisely because I've gotten to know it well enough to appreciate the enormous tactical complexity and non-trivial strategy of the game. And for need to understand the game to appreciate it, *nothing* beats SCA tournament combat. To the inexperienced eye, it's a couple of guys whacking each other with sticks. You have to know the sport pretty well to enjoy spectating it, because only then can you really understand the skill of the combatants.
Really, there's nothing new about any of this: certainly, all of it was just as true in SCA period as today, and it applied to just as many fields of endeavor. Take Renaissance Italian Dance. To the untrained eye, it looks ridiculously fiddly, with the motions so small you can scarcely see them. To someone who has practiced it, though, and especially in period, it was an exercise in precision and discipline, and your entire social standing rested upon both being able to do it and being able to appreciate others in it. Those little fiddly details were *everything* -- exactly as they are in baseball.
So to sum up: how interesting baseball is really depends on two primary elements. One is tribal identification, which is largely a matter of personal choice. Frankly, it's just plain *fun* to get swept up in the tribal spirit for a while, which is why so many people do it when the team is winning. And the other is spending enough time to appreciate the subtlety of the game, so you're seeing more than just stickball. If you get into one or both of these aspects, you're likely to enjoy it; if not, not. Chacun a son gout, and all that...