Because just like with lotteries, there are a lot of blanks, few small wins and a tiny amount of never-work-again wins: Many startups don't net the investor much, some make quite a bit after being acquired, and very few - such as facebook - turn out to be really successfull.
Your phrasing suggests that you compare Facebook (now) with lottery as a system. You should rather see Facebook as a lottery ticket that turned out to be the jackpot.