They're not really. There's a very clear way of thinking if you want to learn jazz chords (though I don't like to call them jazz chords because they're used everywhere). Here's an algorithm that helps you start with chords-
Step 1: learn major, minor, dom7th chords.
Step 2: Harmonize them. Learn Major 7th, minor 7th
Step 3: Extend them. Major 9th, Minor 9th, Dom9th. Move on to 11th and 13th.
This step takes a while-
Step 4: Substitute them. eg- A C major 9th is also an Em9th#5. Wherever you play a sharp 5, you can play a flat 5. So you can play an Em9thb5 or an Em7b5 which is again almost an E diminished. An E diminished is also a G dim or a A#dim or a C dim so where you started playing a CM9, you're playing any of the chords I talked about. That is what a lot of jazz guys do...substitute one chord for the other
p.s.- The 4th step example was rather rubbish. It's just something I thought up sitting in office (and I'm a little out of touch) but theoretically, it should work. Musically, it takes a bit of time to figure out what is nice rather than what is correct. Often, theory is made the other way round. People discover something that sounds nice and then they try to explain why it sounds nice and make it a formula. That way, once you know the theory, you're already progressing because someone else might have taken 20 years to figure out the information you have in front of you.