Why do seerah writers like Ibn Ishaaq sometimes not cite sources for some events?

The source of information of seerah writers like Ibn Ishaq was likely the commonly accepted knowledge at the time, which is why he does not name a specific source.  The main events from the hijrah onward would have been well known to Muslims who lived through that period. Early Muslim society was primarily oral rather than literate, so critical events were carefully preserved through storytelling and repetition. Within about thirty years after Muhammad's death, some scholars had begun to write down accounts of the early history of Islam, perhaps even earlier. Scholars like 'Urwah b. al-Zubayr and al-Zuhri, born in the years right after the hijrah, could have spoken to people who personally witnessed the first decade after the hijrah. By the time they were twenty, they probably had already started learning about this early period from older scholars. Their work involved refining an existing chronological framework, not constructing one entirely from scratch.  Thus, when Ibn Ishaq recounts details of an expedition, he is likely drawing on knowledge passed down and filtered through generations of scholars weighing evidence from many accounts. His information represents agreed upon basic facts, with some disagreements on minor details, not a single isolated source.