Nah open source isn't the answer for everything.
They wrote the software, it's theirs and it works perfectly well and we are already benefiting hugely as a community using it for free! I remember what it used to be like before SO and the alternative sites are just terrible. SO totally transformed things and is just brilliant to use, and free! Go write your own if you want to setup a competing site. These guys worked damn hard, probably hugely late hours while juggling other jobs or whatever to create this. It's theirs and well deserved. Putting it out to open source would be like robbery.
Smart people can't work for free in life. If you compared SO to something in the real, physical world - it would be like the best engineering company building the Tokyo Sky Tree digital TV tower for example. Why can the engineers who build the physical tower in real life be earning millions of dollars, yet engineers who are just as smart, but built amazing "software" still have to hold a day job elsewhere just because some robbers from the open source community want to steal their efforts and replicate it elsewhere.
In order for smart people to keep on doing smart things over the very long term, there has to be an incentive at the end of the tunnel more so than just the prestige of "wow I built SO" that soon wears off if you still need to be working elsewhere at another day job to get by. And this is common place on the internet where heaps of hugely busy sites are run by guys who work for someone else 9-5.
While open source creates some amazing systems and I love it dearly, it's best suited for when the community creates the project right from the beginning - not stealing the result of someone's hard work after the whole thing is already completed and successful.