Like I stated before the reason for only 1 StarPort is imperial control of interstellar trade. Also Scout and Navy bases are listed as being adjacent to the system Starport. A Starport around the gas giant would go go counter to imperial control of interstellar trade.The problem with this is that it excludes all bases (Navy, Scout, etc) which would not be merged with the standard commercial traffic. So there are many systems with multiple 'entry/exit' points that need to be beyond the 100D of various system objects.
I agree with the idea of a large spaceport focused on refining stationed near a gas giant. More efficient, etc. So I can see jumping in or out of the system near that point in space rather than near the primary Starport.