Why exactly does WordPress use p tags, not divs?

You should simply not use <div>-elements to seperate paragraphs from each other. It´s wrong in the semantical context, I think.

When you press “Enter” you make, from a historical point of view, a “carriage return” and then a “line feed”. You start a new line, maybe a new paragraph but nothing completely new.

If you want to start a new paragraph, use a <p>-element. Use a <br /> if you just want to start a new line. Use a <div> if you want to enclose several <p> elements. (Okay, we got <article> and <section> and stuff like that with HTML5, but div is no replacement for <p>.)

The W3 notes: (Pretty interesting document btw.)

Authors are strongly encouraged to view the div element as an element of last resort, for when no other element is suitable. Use of more appropriate elements instead of the div element leads to better accessibility for readers and easier maintainability for authors.

Last resort. Not your “bread and butter and works for everything”-element… 😉