During the time of the Prophet (S), literacy was still new in his society, and his people did not have the custom of writing long books. They also did not have as much technology for writing as Muslims did later; for instance, much of the Quran was originally written on non-paper things like pottery shards. So it was not until later that the major Islamic books were written.
Also, there were a lot of things happening (including the need to handle military threats, the expansion of Islam, civil war, sorting out the practicalities of governing, and so forth), and people are more likely to write books when there is more stability.
It is also quite possible that it didn't occur to some of the people to write a biography. After all, they all knew who the Prophet (S) was, and everything about their society was common knowledge to themselves. Why should they write a book about a subject that everyone knows about?
At the same time, the later companions or those from other regions most likely did not know a lot about the early life of the Prophet (S) because they came into the community later and only saw him in his final years.
Of course many details are passed on in hadith.