The same reason why other doctors study so much, to be honest. The field is a rapidly changing one: new approaches to old problems are found, and shown to be faster or better (hopefully both!) at rescuing the person injured; new tricks for finding hidden problems are found, and can be lifesaving.
The bottom line is that a trauma surgeon has to be as (pardon the pun) cutting edge with things as possible, because time is survival for the major trauma case, and if you can learn a new trick to save a little time, it may make the difference between life and death for the next person you're called to see.
To a lesser degree, this is also true of every physician; I usually log between 50 and 100 hours of self-education a year, just to keep up, and I do a lot of un-logged hours of self-education every year on top of that.
As an aside, there are several states that require a minimum number of hours of additional education every year be documented (education that meets particular standards, by the way) by every physician that wants tomaintain licensure to practice medicine in the state--here in the USA, anyhow. That drives some of the hours of continuing medical education.
Oh, and one more reason: doctors that are worth their salt are fascinated by what we do; we're always trying to stuff a little more in our heads, out of the sheer delight of learning more about a discipline that fascinates us, and that is, in many ways, almost the definition of who we are as people.
MD in practice for a loooooonnnnng time....