We know this because a group of followers doesn’t just form and grow without someone they are following. The entire teachings of everyone within his group revolves around Jesus, and if he didn’t exist then Christianity never began. People need a leader to form a group, they don’t just all be together and say hey, let’s make up a dude and then follow what we made up. They believed enough to die for their group, and they believed enough to dedicate themselves to growing it. That doesn’t just happen if you made it on a whim or made up a key point of your story which could be validated. Additionally just because books were written multiple decades later doesn’t mean that they were completely made up. People can live multiple decades, and stories of experiences and knowledge of people can survive this long easily too. We also don’t know what they knew at the time because lots of information about Jesus that would have survived had been altered much much later by kings who wished to use Christianity for control, we of course know he didn’t completely make up Jesus as there are books referencing him from before this happened, however it does mean our information now is more limited than theirs, so we can’t assume they made this up based upon the same information we have now. It is both logical and just true that they would have had access to more valid information than we do today. They where also often scholars who’s job it was to write about true people and to prove together missing info and validate this sort of thing is even real. With all the information that was available at the time they still believed he existed. We can’t say just because such evidence was lost doesn’t mean that they all came to an invalid conclusion or all made it up. It’s just illogical. As for actual biblical events that had few witnesses, it’s fine to believe that that was made up, or information was lost so it was believed that something happened when in reality there is an entirely different reality that we just don’t have what we would need to piece together today. As we are assuming many stories are being made up or are incorrectly accounted throughout it, it’s not fair to say any events in his life happened based on the book alone, so this a mute point.

TLDR Jesus was real because he formed a group that still exists and followed his teachings from the beginning. Groups don’t just form and follow a fake persons teachings and still believe it and constantly lie about his existence. Scholars closer to his life who had more information than we do now believed he existed, and we can’t invalidate multiple scholars with more information than us just because we don’t have that information anymore.