Because the kind of people that tend to make indie games don't have the money to keep themselves under a roof and fed without taking gainful full-time employment, which gets in the way of getting a game out in a timely manner since it's done purely in free time. The investors don't want to make it into a grant a) out of respect for the developer and transaction and b) because they don't have oodles of money themselves, at least compared to more traditional investors.

Think of it less as a business venture to jumpstart a company and sell it for millions later and more as a way to allow people to have a job doing what they really want to do full-time... so long as the game is successful enough. :)