It is a good question! The 404 status code is useful context. The browser (or user agent, crawler, your code!) can act however it likes in response to the 404. A browser will render the page still, thus all the funky 404 pages since sites can apply their branding to the error. If this were not the case the browser might show a generic message to the user (people would get used to this).

An example of where the browser might ignore the body is in a 301 redirect.