Cpu pre-renders the game code. It'll pre-render those frames as much and as fast as it can. But thats the limit. It ships those frames to the gpu, which finish renders the frames into a picture and sticks it on screen, according to resolution and detail settings. So resolution and detail settings have very little to no affect on fps, coming from the cpu, only affecting gpu output.
With a strong gpu, it'll take everything the cpu can give it and stick it all on screen, regardless of details. With a weak gpu, it'll only render what it can, changing detail settings lowering or increasing fps onscreen, up to the limit of whatever the cpu gives it.
Fps from the cpu is basically dependent on 1 thing. The game code. Games like BF5, Gta:V, Witcher 3, SWBF2 etc are optimized for 8 thread usage. Trying to cram all that into 4 threads is gonna create serious backlogs and prioritizing, which hits fps in the...
Heavily detailed, high physX usage games, massive polygon counts etc all take a hit on the gpu, which is trying to render them. Lowering details alleviates some of that uber fine detailing.
If going from ultra to low changes fps greatly, you have a gpu shortage in power. If it doesn't change much at all, then the fps limit is low, all cpu.
There's only one cure for low cpu fps output, stronger cpu with thread count/IPC to handle the game. If dumping detail settings isn't something you can live with, the only cure is a stronger gpu.
For the Critics: you assume much. In the Indo-Asiatic markets the best selling cpus are 3rd and 4th gen, lga1155/0 is still very much prevalent as 8th and 9th gen Intel is off the charts expensive. So ppl are making do with what they can get.
Even in the States, a used Dell pushing a 4770 is only @ $250-$300 for the whole pc. That doesn't get you anything but a 9400f and the cheapest mobo.
Some ppl are limited by location, budget, availability. Not everyone has options to get 8th or 9th gen Intel. I can't even afford to upgrade my 3770k/970 and I make decent money. Kids come first, along with bills and the mortgage.
So don't judge.
Edit :
At the time of writing, the Radeon RX 580 2048SP hasn't made its way to the Western Hemisphere yet.
Indo-Asiatic market.