Probably because then, it would make your paragraphs 'rigid', whereas if you have visual-line-mode and don't truncate lines — the text would just wrap around — you only need to adjust the width of the window. That works nicely in "distraction-free" modes like writeroom-mode.

I used to fill the paragraphs all the time, turns out — it's really better to leave them as they are, because you can never get the satisfiable number of `fill-column` — the default works in some cases, for others, you'd want it to be wider, etc.