Before I start, I obviously envision there are going to be a great deal of downvotes. This is by no means an attack on any particular religion and I fully support your beliefs just as much as I'd hope you support mine. The Jesus I am referring to is the historical one, as opposed to any narratives about him (such as the New Testament).
I have seen many debates where several sources arise and every single time I have seen it those sources have largely been debunked or proven forgeries (the only instance I can think of is that some people, such as Tacitus or Josephus, having partial non-modified accounts but even then these people are not contemporaries and could have simply written as hearsay), yet it seems that the belief of "Jesus never existing" is simply a theory and considered fringe and that most historians consider it false. Is it simply a religious bias that causes this view or are they actually legitimate? (I have read the FAQ but I didn't find an adequate answer there).
Just as background, could you explain/link to some theories about the early foundation of Christianity that don't involve Jesus's existence?
I can imagine some ideas of varying degrees of plausibility: Paul made the whole thing up and for some reason decided to tell a story of this Jesus guy and his apostles rather than declaring himself the messiah, the apostles created a new religion by committee, there was some sort of movement among Jews dissatisfied with the Pharisees, Sadducees, and Essenes which created Jesus as a sort of allegorical "wise rabbi" figure which spun out of control. However, I don't know what the proponents of the nonexistent Jesus theory regard as the most likely explanation of Christianity's foundation.
I suppose it's just easier to imagine that Jesus was a person, like Joseph Smith or Ellen Gould White or Baha'u'llah, than to think that the actual founders of Christianity created this character out of nothing.
Okay, I want to step aside from the body of your text and discuss your title question first. Why don't historians take Jesus Mythicism seriously?
The answer is because it's not serious history done by serious historians. Most people writing claims about Jesus not existing lack training in the work of ancient history. They lack solid knowledge of Greek, Latin, Aramaic, Hebrew. They lack an understanding of historical process. They don't hold degrees in appropriate fields. THey produce work that is not peer-reviewed and not scholarly rigorous. And in this area, most of their arguments are not really new arguments.
So historians, and historians in the fields that relate directly to Historical Jesus, generally don't waste their time because these positions are on the fringe. This is not a religious bias, plenty of non-religious scholars in the field think that Jesus is a historical figure.
I would recommend you read Bart Ehrman's book on this, Did Jesus Exist?, precisely because it is a book written by a historian in the field who deals with major proponents of the "Jesus never lived" hypothesis.
Okay, to return to the body of your text, what sources do you mean that "have largely been debunked or proven forgeries"? It sounds like you think the New Testament is inadmissible data in this debate, is that correct? Why do you think this?
If you do a search for "Jesus" in this subreddit you'll find hundreds of previous threads discussing this question. It comes up every other week or so.