[Context: an assertion that split-screen is irrelevant because people can buy a PC and a car and go to a LAN party]
I don't think that was the assertion at all. It was that split-screen is more of a niche because most people have a PC or a laptop or a console and can transport this somewhere or have friends transport theirs.
So what factors have driven the slight shrinking of the split-screen gaming segment? Well the growth in the PC market and the slowing of the hardware race means that most people have access to a computer that can reasonably play most games, the increased pervasiveness of low-latency internet connections means direct physical connections (even LAN) are less necessary now and the reduced cost of TVs mean many households have more than one already.
Despite this, a huge amount of AAA titles still support local co-operative/competitive play. Pretty much all fighting and sports games do as do hugely popular AAA games like Call of Duty, Borderlands and Gears of War.