The reason the investor wants to give the founder an early earn-out is that they don't want the founder to be defensive. If you've invested $45M and you're looking to make ten times that, the site needs to make it really big. And you don't want to have a founder looking for the quick win. You pay him/her an amount in the ballpark of what is often colloquially phrased "fuck you-money", and it increases the chances that he's willing to risk everything to give you that 10x. For the investors, it's a calculated risk.
