HELP!! Confusion on PWM Y-Cable

Hi everyone! I am the beginner of playing DIY on my own computer. I have searched the PWM Y-Cable in the market, they are only using in the mainboard. The PWM signal input from the mainboard is used to control a series of connected PWM fans for optimal performance in noise and cooling.

Now I come up some questions that need someone can fixed my confusion.

1) Why the PWM Y-Cable just used in mainboard, not in graphic card?
2) Is it necessary to input the signal from the GPU and make the RPM of VGA cooler and case fan at the same speed?
3) If it makes sense, why I cannot find the PWM Y-cable for graphic card?

That’s all I want to know. Thanks~