An infinite baffle supposedly requires double the drivers because you are throwing away the reverse excursion? This is what I've heard at least. It's supposed to sound cleaner also as a result, because you never deal with the reverse excursion, which is not going to be identical to the forward excursion.
As JimP mentioned, all drivers have a harder time reaching higher SPL at lower frequencies. So you either need a lot of surface area to move back and forth a little (multiple drivers) or less surface area to move back and forth a lot (greater excursion). Since drivers are physically limited in their excursion, multiple drivers is a simpler solution.
If a driver attempts to move back and forth past their physical limit, that's called bottoming out and you'll hear clanks or clacks. That's okay if it happens once or twice, but if you keep pushing your sub that hard you can do permanent damage. I'm led to believe that a sealed subwoofer cannot bottom out because the sealed volume within the enclosure will prevent that, if properly matching the enclosure, driver, and amplifier. But I'm not entirely sure of that.