rendering – Why do games run slow on CPU

Maybe my question sounds naive because I’m not a game developer but I dont see objective reasons for this.
An average cpu is in times more powerful than an average gpu like a few graphics cards in one.

The most repeated reasons in internet:

the GPU isn’t as busy as the CPU

If the gpu renders the screen how it can not be busy
The modern cpus are powerful enough. They are not busy most of time. For example my cpu’s usage is 3%.

many GPU’s are designed to process graphics commands in parallel, CPU single threads them

What bothers the games to run a few parallel processes even on one cpu core?
In my opinion its not correct to tell that the gpu is better if games never use 100% of the cpu’s capacity. To be specific they dont use even 50%.