To expand on your point, if one was to sustainably grow all vegetables required for a healthy, sustainable diet, then currently this can be achieved on around 100-200m2 per person (on earth, requiring no extra fertilisers / soil etc) based on research by John Jeavons. This hasn't taken into account external rainfall or air movement, but the basic premise is fairly well established. Throw in water treatment / recycling and humanure composting to close the circle completely, then all you need is a light source (assuming a closed atmospheric situation....)

Assuming that we can compress that down into 50m2 per person, the spatial requirements for a sustainable colony blow out quickly to large proportions, and we don't have information over the long term of just how well such a closed environment can nourish a small population - there's literature and research suggesting that we must have animal products in our diet for long term health, so that quickly adds up.

Given all these factors, supply runs from a known good source (earth / the mother country / home) are a sensible choice - but once you start roaming much further, it isn't an option any more. This is the stage we'll find ourselves at in 10-20 years time, as space travel will be economical to the point of sending out pioneer crews to space.