There's no reason to not have better CPU's. If we all still had 9800gt's and core 2's, Crysis 3 wouldn't have happened in the first place anything like it is now.
Aside from that, multithreading is hard. It takes more work than single. Because of this, lots of game and program developers don't have perfectly multithreaded code, which basically means on all of the games that have a lot of work done in the main thread, such as Starcraft 2 and Planetside 2, performance is terrible at times. I could say, for example, that i wanted close to 200fps most of the time for a 144hz screen - but Starcraft 2 and Planetside 2 frequently pull the best systems on the planet below one fifth of that.
Even if a game was perfectly threaded, there's plenty of stuff you can do with more CPU power. Battlefield 3/4 have player counts limited to where they are not just for balance, but for performance reasons. RTS type games could have much more complex unit interactions and more units while running much better. The possibilities are pretty crazy.
There's also live encoding. Currently, on a 4770k, for sc2 at least, which leaves ~2.6 of your cores idle - you can max out CPU with 1920x1080, 60fps@veryfast preset x264 live encoding. Using a better encoding preset at the cost of higher CPU requirements, to increase quality without increasing bandwidth requirements, would raise CPU requirements by 2-3x to go to Medium preset, which gives gains that anybody would want.
x265 is coming, and while allowing MUCH better quality at the same bitrate... the team hopes to live encode 1080p@30fps on an xeon this month i think it was from tomshardware preview of it.. good luck running CPU heavy games and livestreaming @1920x1080, 60fps
Also for this thread: We knew Ivy Bridge was single digits percent higher IPC than sandy bridge a long time ago, why so many people freaking out over it? What is VERY interesting is the pricing, the solder, the overclocking performance, potential, heat, etc