Because people like me are willing to spend the money. I bought this year a used 7700k for 220 € and 4790k for 175 €. It is way less hassle to just switch the cpu. The price is also much less when selling your old cpu
No need to spend more on a Ryzen build when the old stuff does the job as well for cheaper. I can buy a new system in a few years instead of upgrading ryzen