About number of reviews on Amazon

Hi

When I watch videos on how to find niches on Amazon, I hear people saying that there shouldn’t be to many reviews.
I assume it’s a competitive thing, but I don’t see why 1 product with 1500 reviews should be more competitive than a product with 245 reviews.

Is it simply because that with 1500 reviews, the product is very popular and therefore very competitive?

Or is there more to those reviews?

Thanks