Because at 4k with 4090 you get 5-8fps more on average from 13900k over 13600k. Pricewise 13900k is around £540, while 13600k is around £300. Even if we make it 10fps more, that's 24 quid per one frame, which is obviously a bad value. With 13700k difference is even smaller. Yes, only 100 euro more, but those 100 should bring value - 13600k is not gonna last less neither it is substantially worse, we are talking about the same gen, where 600 and 700 are close to identical. If you want to get value, spend those 100 on DDR5 - now THAT will last longer as you'll be able to switch CPU in a couple of years without buying a new motherboard and use better RAM (atm there's not much gain between DDR4 and DDR5 RAM, but with time there will be big difference, the same way it happened with DDR3 and DDR4). Long story short, 13700k will bring diminishing returns over 13600k, just check some benchmarks, especially if we are talking about 4k, and you can reinvest these 100 usd into something better.