Bounty cap and bounce back potential

So yesterday I was in a match where my team was steamrolling the enemy when a thought occurred to me, "why does the bounty rewards cap at 500 gold?" and after thinking it through, I could not for the life of me come up with a good answer. I mean at that point in the game many of my teammates and myself could get away with a number of plays for little cost, and in the end the difference we could make doing a risky action in a battle far outweighed the downside of it failing and giving the enemy team a shutdown.

As far as I can tell the bounty element was implemented to give bounce back potential to a team that is struggling against fed champions. By working together and awarding teamwork and taking advantage of your enemies mistake you would be able to secure a high threat kill that could give your team some much needed gold for a comeback. This part makes complete sense, it awards the underdogs for cooperation and punishes the overly fed from getting to cocky and making a mistake. But the thing I can't seem to wrap my head around is why the bounty is capped at 500 gold.

At a champions 5th kill they are now worth the maximum bounty of 500 gold, every subsequent kill at that point adds absolutely no more value to killing them, but in the meantime every kill they get past this point just adds more to their own gold advantage while making sure their own worth as a target remains static. Why is this? I am not saying the value should increase by a large increment by any means, but wouldn't it make sense if it did continue to go up?

I realize that the later we get into the game the less prevalent bounty becomes, because at that point losing out on a teammate is large enough deal on it's own, but when struggling to keep up in the early-mid game every little bit of gold is important to building up essential items for that transition later on so I don't see why the county caps so quickly. Even something as simple as an additional 10 gold per additional kill could make a difference against an overly fed champion.

In the end, it isn't exactly a major change. It isn't like it is anything big enough to change the game drastically or needed in any way, just a fun thing to think about. What about you guys? Do you have any idea why the bounty bonus is capped?