I'm going to try an answer I don't think anyone else has given.
They tried founding colonies but they kept failing until the residents of the ship gave up
The shipborn have been traveling for a long time. The origional intent for their mission was that they would seed colonies.... but that was long long in the past.
The initial journey was a long one, while on the trip over many many generations the shipborn gradually became more integrated with the ship itself. Often in ways they're no longer even aware of. Every Shipborn carries in their blood and every cell of their body nanotech, their bodies are supported and regulated by implants that interface with countless ships systems. They even have biological technology integrated at every level that was designed on the ship, tested on the ship and implicitly relied on many conditions within the ship. Some of it they don't even think of as technology since it passes from mother to child in the womb.
Biotech motes are even carried through the air from Shipborn to Shipborn carrying updates related to new pathogens sourced from ancient distributed ships systems.
Indeed the full understanding of some of the technology integrated into their bodies has been lost with time.
The founding of the first colony appeared to go well, while the ship remained in orbit and colonists were still regularly traveling between the ship and the surface. Indeed nothing started going terribly wrong while the ship was still close enough for ubiquitous networking to connect the 2.
But when the ship reached the edge of com rage they started receiving frantic signals from the colonists. They were succumbing to local pathogens that had previously posed no threat, their organs had started failing in unpredictable ways and colonists had started developing deadly autoimmune disorders.
By the time the ship had slowed it's course, turned around and re-entered orbit everyone on the surface was dead.
The third colony
This time the ship remained in orbit for several years to keep watch and to provide rescue in case of disaster. In all the time they waited the colonists remained fit and healthy.
But again shortly after final departure the distress beacon lights up again....
The 10th colony.
A few brave volunteers tried a final bid to survive on a planets surface away from the ship. They had countermeasures to try to protect themselves from many of the countless hypothesized causes of the previous failures. They sealed themselves inside pathogen-proof domes, they had satelites searching for possible hostile alien bioweapon incursions. They had supplies of safe food and weapons... but they didn't have duplicates of 7 of the 23 separate software and nanotech systems that the Shipborn had unknowingly come to rely on to maintain life.
And so they died like all the others.
Now it is accepted that god and a hostile universe does not intend for the Shipborn to live away from the ship. The ship is life
Sometimes a young rebel or skeptic will try, will ignore the warnings of their elders and insist on remaining behind perhaps with a half dozen of those like them. They always die.
Over-dependence on technology they no longer fully understand.