No scholar has ever been able to produce a single credible theory on why Luke may have wanted to fabricate his story. There was no media fame waiting for someone who claimed the heresy that a person was God – only persecution.
Additionally, Luke provided a list of names, local customs, and other details which would have only been possible had he witnessed the events he described. This wasn’t published information and he couldn’t look it up on Google! He had to be there. And, because his account was written while other witnesses were still alive, anyone from that region could easily refute his story, if it was made up.