My opinion: because here you can find arguably the highest concentration of powerful and knowledgeable minds, and this is the sort of thing that the world needs working on these hard problems.
I find the concern over serious issues that is often expressed here to be rather disingenuous, as people love to engage in bitch sessions about the bad behavior of the members of their outgroups, but the notion that we should consider rising above that level of behavior and think about trying to find actual solutions is somehow ~inappropriate, as this is a place only for "curious conversation" (except for the exceptions to that rule, of course).
