"why so many drivers in some speakers?"Depends. Reasons vary.
The current reigning champ is Eric Alexander at Tekton. In studying the physics of sound Eric realized the moving mass of a lot of instruments (violin or guitar strings, etc) is very low, often on the order of a gram or so. The typical moving mass of a midrange driver on the other hand is often times an order of magnitude greater. Eric reasoned it is difficult bordering on impossible for that mass to accurately reproduce the harmonics and timbre unique to each instrument. From this he had the insight, low mass tweeters can do this, and if enough are used will reproduce midrange better than one large midrange driver.
So that's one. One down. Who knows how many more to go....