Tacitus said on the Germanic tribes

"In their ancient songs, their only way of remembering or recording the past, they celebrate an earth-born god, Tuisco, and his son Mannus, as the origin of their race, as their founders. To Mannus they assign three sons, from whose names, they say, the coast tribes are called Ingævones; those of the interior, Herminones; all the rest, Istævones." Source http://www.perseus.tufts.edu/hopper/text?doc=Perseus:text:1999.02.0083:chapter=2&highlight=

As we can see, there ancient songs were the only way they passed down history. As they were not literate, their record were not very well passed on. This is why many of the accounts come from Roman conflict.

Pytheas is not the only person to explore Northern Europe before the rise of Rome, but he was the only to record the Germanic people's culture and traditions pre Roman era. His works, however, did not include history.

The tribes also had cultural exchanges with Rome, which took away some of their traditional song histories. This, coupled with permanent settlement and the arise of more individual cultures among the tribes, and the fact that oral history is not the most reliable for recording history, led to the histories of the tribes being lost through time.