How come tele-convertor must put on between theTele Lens and Camera body?

One some cameras, they do connect with a bayonette mount to the end of the lens - but that is pretty well reserved for twin lens camera and very old range finders.

If you have a built in light meter, you should have no problems with the one that connects to the body and then the lens. If you do not have a light meter, you will have to open either 2 or 3 stops, depending upon the lens.