Lots of smart people work here. The pay is pretty much as high as you could as for in the IT world.
...There are many interesting problems to solve. There is never a shortage of work to be done, and being on the cutting edge of high frequency trading is a pretty cool gig, even if the work has no greater societal meaning. Don't worry, the pay will make the ultimate meaninglessness worth it.
Oh yeah, the meaninglessness of it all. Who cares if we trade faster? Doesn't do anyone any good except for our own. All we really do comes down to skimming money off of less sophisticated traders, taking advantage of market inefficiencies, and beating our competitors. It's a rat race with no clear goal. We don't provide any value to society, only leech off of it.
...It's more of iron sharpening iron, and I've noticed that the people that learn how to navigate this... erm... heated environment... tend to come out of it as some of the absolute best programmers and sys admins on the planet. If you can make it here, you can make it anywhere.