The part that baffles me a bit is opposing slides that are only around 20" deep. Our single slide is 42" deep.
We get the same interior space (open or closed) but only have one mechanism to maintain.
I suspect some of it is marketing. It sounds more impressive to claim 8 slides even if they are of limited benefit.
Side Note: walking into work today past the new superman movie shoot. They have a parking lot turned into a food area for the crew. They have a couple of semi trailers converted to dinner cars with 4' slide outs that run the length of the trailer. I peaked inside and the space is just incredible.