In theory, the chains may have equal strength in some respects, but that's only one part of the drivetrain. You have to look at the system as a whole to see how the parts interact, where the inefficiencies lie, where the benefits are gained, and how the system can be improved as a whole.
It may not be the chains that are holding back the system, but the rings, especially in the larger sizes. This may be where going going to 3/32" is not a good idea, hence people staying with 1/8".