why do you say math instead of maths??

Australians say maths too.

Most Australian students would not know what you were talking about if you said you were going to your math class.  It would sound really weird.  

When we say maths it often sounds more like mass.   We are a lazy bunch.    LOL

1) Life is too short to say and write all those extra s's. (After all, we have 'color', not 'colour'.)

2) All of math is interconnected in some zen-like, gordian knot way.

Math is the North American abbreviation of mathematics. Maths is the British abbreviation of mathematics. The British usually abbreviate by including the last letter of the word as well as the first few. This is why the abbreviation for mister is Mr and for missus is Mrs; thus maths.

By the way, the Americans abbreviate mister with Mr. including a full stop, while the Brits only use a full stop at the end of an abbreviation if the last letter is not included.