there is no source it is pure conjecture, but I would say that there are many many fine tunes available of various image generation models so it is clearly possible to make many styles. thus it must be a conscious choice on the part of a API provider to render by default to a distinct style. there are many plausible reasons they would want to do this. Surely, but I don't have actual evidence from the internal management processes of these organizations that they were doing this for one reason or another.
It's industry standard procedure to tune your model to output a consistent distinct style, to prevent malicious actors from abusing it and presenting fabricated (but very convincing) images as real.