Why when sending data over AJAX, do you have to JSON.stringify() your objects?

AJAX is all about HTTP requests, which are basically "text" requests to a server. That's the main reason why you have to stringify your object: That way it's turned into text that can "travel" over HTTP.

when sending json via ajax do you need to turn it into a string to send it?

If it isn't a string, then it isn't JSON in the first place.

JSON is a text based data format. HTTP is a text based communications protocol.

JSON stands for javascript object notation

JSON is based on the syntax for JavaScript literals. A JavaScript object is not JSON.