Most Space Opera envisions interplanetary trade as just a larger scale version of planetary trade(and tbf: this is because most space opera is exploring real-world politics allegorically rather than being genuinely speculative), but the reality is(barring the invention of large-scale teleportation, which would put an end to the shipping-trade anyway) it is just SO MUCH Easier to grow all the food for a given population on that planet than to ship it in from somewhere else. Even if we're talking Ecumenopoloi(EDIT: Planet-wide Cities, not city-wide Planets >:T Thanks Brain), a few relatively small "Gardens" maintained in choice regions would do a better job of feeding the populace than shipping produce from off-world.













