The fact that they weren't written until several decades AFTER the events described, the fact that they appear to be copies and/or based off a single source document, the fact that they contradict each other...
The book of Mark is suggested to have been written by 65 AD, which means Josephus would have been in his 30s.
James, brother of Jesus is said to have been executed by 62AD. Josephus again would have been some 30+ years old and his works came out perhaps just 30 years later.
We aren't talking about multiple generations here, these historians were living at the same time as the apostles.
I didn't witness the Vietnam or gulf wars first hand, but that doesn't mean that I'm not a credible source on whether or not they occurred simply because I wasn't there to experience them first hand, or simply because I wrote about them 30 years after they occurred.
Remember, Jesus was just a nobody rebel carpenter with some fisherman followers. There is no reason to believe Josephus or Tacitus or the historians that perhaps taught them or raised them, would have even cared about Jesus during his ministry. Nobody of course would care to have written about Jesus until after he was executed, and that's what we see. Within a single generation after the alleged execution of Jesus, we have the apostles generating their gospels, and a Jewish and Roman historians talking about it, which is just what we would expect.
Also, remember further that, given that Jesus was hypothetically just a carpenter, he would have had a hired professional scribes with him during his ventures. So there is no reason to believe that anyone necessarily would have written down his history as it was unfolding, no more than is the case for you or I.
And while it is true that even the gospels sometimes contradict one another, it is also true that in many cases, the gospels tell the same stories, and mention the same people. The same people who also happen to be spoken of by these historians.