I am not an expert, no you dont have to point me to my reputation here to prove that, I know for a fact that I am not, the reason I am answering is because this could be useful to attract experts as reasons that others here may not have thought of. But again, as an outsider (to the "experts" group) there is always a chance that I could be wrong.
First reason (Selfish to the expert) - There are many people using stackoverflow (referred as SO) site as a proof of their expertise, authors, consultants all kinds of people (Example:https://stackoverflow.com/users/1144035/gordon-linoff). academic expertise is one thing, ability to apply to practical questions is another, and so as a practical online resume this is helpful to some experts
Second reason (Wont work until the community grows) is that people who are at the top of the Maslow's pyramid may actually look at this as a social service, when this SE grows (which SO has achieved) people can see this platform as a place of choice to help people
Third reason (wont happen until this becomes very large) - this platform, if and when it reaches a size where most experts in this field would deem it necessary to participate here for one reason or the other your reputation would become your identity. Someone with 100000 reputation in SO is an expert, but someone with 400000 is at least seemingly better and hence becoming part of this community at the earliest gives them the edge