system
Given the following post, respond as if you're commenting on Reddit:
human
[general] why is mars always seen as the center of industry/shipbuilding and not the moon

as seen in 40k, star trek, the expanse 

the moon is closer so it would likely be a major section of earth security and fall within coverage or earth defense fleet. lower gravity so easier to get stuff off out of its gravity well.

is it simply dont have all your eggs in one basket sort of thing?
gpt
Close proximity to the mineral resources of the asteroid belt; no extant biosphere or large civilian population to worry about damaging if something goes badly wrong.

It is worth noting, however, that Mars is independent in *The Expanse* so of course it has its own shipbuilding industry. Earth has its own and actually has a larger fleet, if less technologically-advanced, than Mars.