[sundarurfriend]

That's an interesting point (no idea why you've been downvoted for it). I have a few theories why that might be:

1. Groups like these were more likely to survive if they were isolated and not occupying territory that the (presumably) more sophisticated Homo Sapiens coveted, so maybe they just didn't get interacted with otherwise. Rama had a Vanavasa (forest-living) and a generally weird out-of-pattern life for the time, so had a slightly higher chance of encountering them.

2. If people in later stories did come in contact with them, they were probably trying to expand their territories or otherwise conquer the land. In that case, the stories are going to mythologize them as savage Asuraas, to give further justification to the conquests.

3. We only have a few stories, of unknown time origin, remaining out of hundreds of thousands of stories told by different groups in different places over the millenia. So even if a significant portion of the stories featured Vaanaraas, it's plausible they might all have been lost before they could reach us.

4. In particular, given that such groups were declining in numbers over time, and the fact that more recent stories are more likely to survive, it makes sense that stories featuring them are more likely to have been forgotten and lost to time.