Realism has little to do with it.
Those products can be produced only on planets by design because of gameplay reasons.
CCP could easily allow players to produce gas or ore from planets. Or they could let players find fully made industry items in data and relic sites.
But no, such things would make respectively ore mining, gas mining and regular industry a lot less impactful when there are alternative sources for the exact same items. For CCP it would be a balance nightmare, because whenever one source is deemed ‘easier’ by the players or ‘cheaper’, the other is ignored and people ask why no one does PI, mines or or does industry.