For the same reasons that you might donate time or money to anything. Why should you tutor some disadvantaged kid for free without having an agreement that you get a percentage of future earnings? Why pay taxes or vote for the continued existence of taxes?

Because donors turning a profit isn't the point. Most startups fail. Getting a percentage of something which is probably going to turn out worthless is less important than shifting the ecosystem away from one in which promising startups are destroyed by trolls.