Thats interesting. My 5 year old Ryzen laptop can transcode 4k faster than realtime, which is what I mean mean by "these chips". Modern Ryzen, which is what the subject is about.
Quick Sync is invaluable for low powered processors, my old Intel embedded Wyze can do several streams.
"faster than realtime" doesn't mean much if it's in a device that's supposed to do more than just transcoding (such as serving a web app) or if you need multiple transcodes, etc.
Even on modern chips, transcoding is quite expensive.
People who are running Plex generally are running on servers also serving files, web apps, and who knows what else. These devices are often running 24/7, so both overall cost and power efficiency are big concerns. I wouldn't want to rely on my server being at high CPU usage most of the time - for power, heat, and overall reliability concerns.