This is probably an unpopular opinion, but here it goes…

Because the Hero’s Journey is junk science. Joseph Campbell who created the “Hero’s Journey” was an arch-reactionary who had intended to present an alternative to the literature of his day which he disdained. He longed for a return to the “wisdom of the past” and his book “The Hero with a thousand faces” was positioned as presenting this wisdom, arranged around Freudian and Jungian theories, particularly the debunked “collective unconscious”; as well as the reactionary philosopher Spengler and ethnologist Frobenius (particular Frobenius’ idea of “paideuma”).

What the book actually is, is a mashup of quotes from various myths and legends, deprived of their cultural context and strung together along the lines of Campbell’s preconceived beliefs. These myths and legends are never considered in their entirety, nor did Campbell dwell on the culture that produced them. From a standpoint of anthropology, it is a profoundly unscientific book and upon publication it was largely dismissed as such by Campbell’s peers.

In the “American Anthropologist” Stephen Porter Dunn wrote:

Campbell’s book is in a sense a throwback to an earlier heroic age of anthropology, when the air was dark with flying hypotheses and comparisons rained down like acorns in autumn. Reading it, the case-hardened social scientist derives the same sort of nostalgic, half-shamed pleasure as the ordinary adult would from reading G.A. Henty or Robin Hood to his children. Campbell uses the traditional equipment and methods of a literary critic, for whom comparison and analogy are tantamount to proof and fact. He writes in a curiously archaic style - full of rhetorical questions, exclamations of wonder and delight, and expostulations directed at the reader […]

Accordingly, The Hero’s Journey always mattered more to literary applications than as a genuine human artefact. Though the importance of the The Hero’s Journey as a storytelling concept only came after the success of “Star Wars”, when Hollywood discovered it as a template for blockbuster movies. Though Hollywood largely discarded most of Campbell (for good reason) in favor of the simplified version as presented by Christopher Vogler (for example: The 12 steps of the Hero’s Journey are Vogler’s version, Campbell used 17) who did not care much about the Hero’s Journey allegedly universal applicableness, than in its usefulness as a tool set.

Lots of people are still heavily invested in painting the Hero’s Journey as a universally shared principle of storytelling. To those people I recommend Robert Ellwood’s “The Politics of Myth”, which traces the development of Campbell’s thinking and the development of the Hero’s Journey.