Even in terms of mechanics, consoles are lousy for FPSs: controller vs. K+M; the mouse always wins. From a PC-superiority perspective, the best way to do an FPS is therefore Keyboard and Mouse, which means one player sitting in front of a screen. Consoles can't beat PCs on technical specs.
The result, someone who wants a "serious FPS" is going to do it alone in a darkened room in front of the same device that delivers pornography.
Consoles, on the other hand, are hooked up to huge screens and are played on couches. There are often other people around, which is what can drive sales. So, yeah, split screen makes more than sense, it makes sales.
Of course, the way all consoles are selling now, their target demographic is fast becoming married men who only get to play for an hour or two late at night after the spouse and kids have gone to bed.