Actually, majority/fair bit was written within the expected lifetime of the people involved (including Apocrypha here) or within a direct relative/friend/child's lifetime.
Countless reasons.
Bible includes personal accounts, personal letters, age-old-retelling's no-one could know, children of the author's retelling's etc.
Consider, this was a renaissance for all purposes, and when people were getting oneven before thatpeople wanted to put that down to text for history and sourced from a great many things…
The personal account I'm thinking of is an account of the Gospel of Matthew(?) being taken by the Apostle's on some early missions, presuming because it was one of the first New Testament texts to be written.