No particular reason, other than that the database is pretty big, and we haven't gotten to it yet.
The list of things that needed to be fixed from stock DW was in the hundreds, so that was pretty far down the list.
A project is currently in the works to recraft the whole database, however, I am not directly involved in the work at this time, so it is not clear when this will be available.
If you are interested in tuning your own database, there is nothing wrong with changing the weapon effectiveness of missiles to something less than 100, it functions as a simple percentage chance of failure when the missile is otherwise successful in intercepting the target.
Cheers,
David