Remember the days when there was actually two GPUs on the one card? I had two of those AMD cards, Radeon 5970 and 7990 .... in those days the game engines were simpler (as explained by Fellix above) and could do AFR and other techniques, now the game engines are not compatible with splitting across multiple GPUs. Also as others said, the increase in framerate in those days of using two cards was completed invalidated by stuttering which made it look like less framerate than just using a single card. And then there was the games where MSI Afterburner would show that one GPU was at 100% usage and the other was sitting at 0%. I'm sure using "chiplets" to scale GPUs will cause similar game engine problems as SLI did. But maybe the massive bandwidth increase of PCI-E 4.0 will help (or the interconnnect between GPUs similar to the 3rd Gen Ryzen chiplets). A lot of it is economics too... NVidia would much prefer people had to buy its more expensive GPUs than have any option of doubling up a much cheaper or second-hand card later on to get same speed.