Why inline CSS styles on images?

I am having some trouble configuring config.allowedContent = true. It works the first time I use it on a piece of content in Drupal but eventually reverts to the default behaviour which is to strip height and width attributes form images and place them in an inline style declaration.

I don't understand why this would be the default behaviour. The width and height attribute are actually preferred for performance reason and inline style make it difficult for images to be responsive.