Why is there bonus gold when multiple people particpate in a kill?

For what reason does someone deserve BONUS for assisting in a kill? Why is a kill that involved multiple players more valuable than a kill that only took one?

First blood gives bonus gold to reward the team/player that pushed for the advantage but what exactly is the point of bonus gold for cooperation kills? I feel like this plays a much bigger role in why bot lane and jg are so overtuned right now and why assassins are in constant motion of being in or out favor in the meta.

The game is actively punishing solo players actions and UNFAIRLY rewarding teamwork. Either give solo plays/actions the same or similar reward or take it out of the game.