Because the original purpose of floats is not layout (where you want the thing to stay in the other thing) it's typographic floats, like pull quotes or info boxes or pictures. In that case, even if your float is in a paragraph you don't want to "expand" the paragraph past the end of the float if the float is longer than the paragraph, you want the following paragraph to be laid out around the float instead.
That floats were press-ganged into layout does not mean their basic behaviour is bad, it just means they were used for something that was never intended (due to a lack of actual layout tools in CSS until CSS3) and they were not very good at it.
And note that the lack of layout tools was not limited to "applicative" layouts, most papers are laid out in multiple text columns yet that fairly simple and common layout was not supported either until CSS3.
Oh, I see what happened. I have a minimum font size which makes the first paragraph tall enough so that it wraps all the way around the image -- so the clearfix is effectively redundant with my font sizes. The image clears it up, thanks!
Here's a demo: http://codepen.io/anon/pen/ggGgWb
Here's the "clearfixed" version which is nonsensical: http://codepen.io/anon/pen/xgXgry
That floats were press-ganged into layout does not mean their basic behaviour is bad, it just means they were used for something that was never intended (due to a lack of actual layout tools in CSS until CSS3) and they were not very good at it.
And note that the lack of layout tools was not limited to "applicative" layouts, most papers are laid out in multiple text columns yet that fairly simple and common layout was not supported either until CSS3.