That is a specific implementation of image models, not true for all image models. For example Civitai has a lot of realistic models that look like people took them, at least on first glance, as I'm sure with enough pixel peeping you can find discrepancies, point is the "flatness" look is with that specific ChatGPT model.

For example: https://civitai.com/models/2149369/iphone-realism