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.
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.