There's also a technology that uses narrow band filters and different rgb for each eye. The really simple answer to projectors for stereo (please 3D is something else entirely) boarding two by two is you throw away so much with any of these techniques that the simplest way to fill a large screen with enough photons is to use more projectors.
"There are painters who transform the sun into a yellow spot, but there are others who, with the help of their art and their intelligence, transform a yellow spot into the sun." Pablo Picasso