Because PCs kept getting faster for the next decade... and software kept requiring the faster speeds. Once we got to the point of the Intel Core Duo (06-08) the modern processor was fast enough for the majority of the PC fleet. You can take one of those 8 year old PCs and install Windows 10 on it and it will work as long as you have at least 2gb of memory in it. Also, PCs at the time didn't have much horizontal competition yet, notebooks were still pretty expensive, smart phones weren't smart, and tablets didn't really exist.

The current and previous generation or two of tablets do what most people ask of them. Facebook, Youtube, record some crappy video, all still work. If the tablet stops working the user might just stick with their new smartphone with a 6" screen.

> Once we got to the point of the Intel Core Duo (06-08) the modern processor was fast enough for the majority of the PC fleet

Core 2 era was definitely the change-over point for me - before that I was upgrading pretty much on a yearly basis. I then built a Core 2 Quad machine in mid-2008, and stayed with the same hardware until mid-2013. TBH, that upgrade (to a i7-4770K) wasn't really necessary - I only did it because I started doing a fair amount of HD video encoding/editing. The Q6600 was handling everything else fine.

After the Core Duo we got integrated graphics, built-in video (etc) support, and lower TDPs that allowed thinner laptops with longer battery life. Oh, and lower prices.

We also got 10-second start-up times via UEFI.

There are more things to PC life than raw single-core performance ;-)