The short answer is that many written accounts aren't necessarily reliable--and we've come to understand (more recently) that many historians made things up if reality didn't conform to their world view. Actually, the "make things up" still continues to this day; most popular history books fall prey to this.
In the past 60ish years, we've started to make good strides in rebuilding our understanding of history based on firmer grounding of reliable facts and evidence, but where we lack reliable archaeological evidence to backstop our historical narratives, we still end up relying on plausible lies. South American archaeology is much less thoroughly explored than North American or European archaeology, so we've had less ability to revisit what we thought we knew. We are learning more, but this is very recent (21st century recent, far too new to really percolate into the popular history sphere).
In the past 60ish years, we've started to make good strides in rebuilding our understanding of history based on firmer grounding of reliable facts and evidence, but where we lack reliable archaeological evidence to backstop our historical narratives, we still end up relying on plausible lies. South American archaeology is much less thoroughly explored than North American or European archaeology, so we've had less ability to revisit what we thought we knew. We are learning more, but this is very recent (21st century recent, far too new to really percolate into the popular history sphere).