Did he not separate the authentic from the weak like Bukhari, Muslim, and others did?
Ibn kathir(r) was more than 400 years later than Bukhari, Muslim etc. and he was not an expert of that level like Imam Bukhari(R), Imam Muslim(r) etc.
Another IMPORTANT fact is, in the case of history , authors tries most to gather the most Data.
Ibn Kathir(r) mentioned conflicting,differing narrations(incidents etc.) in Al Bidaya wan Nihaya with the name of the narrator/source- making a chance to examine for those who wish.
In several cases, he also commented.