Admittedly, you are no expert. Neither am I, but the question I have is:

How often is the gun used to strafe ground targets in the first place?
If
the gun isn't used much, there isn't much point to wasting the
space/weight,
is there?


The same sort of reasoning was applied to the lifeboats on the
Titanic......

which weren't much use as most people didn't get to them in time anyway...