Why in professional games isn't farm handed out to other laners when full build?

I sometimes see an ADC with 500+ farm in a long game, and a Karma support with 3-4 items tops and 50 farm still not being hand over some farm. Why do ADC's still take farm when they made over 20k gold anyway, as they can't buy anything else, and already topped with elixirs.
Report as:
Offensive Spam Harassment Incorrect Board