Generally because top of the line computers are expensive as hell. If the one and only thing you use your computer for is gaming, you would not pay extra and just use the cheaper part.
But, if you are doing multiple things with your computer. Such as work related tasks that benefit from more cores, as well as want to have top of the line gaming experience and can do that off hours without interfering with the purpose of the computer for work related money making activities, then the route to go is to buy the premium part that makes 1 machine fit both roles.
Top of the line graphics card, $2000, best of the best memory who knows, mine usually costs $200, top of the line motherboard, $400 to $700, A few high performance NVMe drives, $200 to $800 a pop. Spending the $280 extra for the 16 core part can save you thousands of dollars and valuable office/game room space and the bother of pushing extra buttons to switch between computers.