The answer to your question is that people have become 'lazy' in their word usage and they just skip the S on the end of maths
It has nothing to do with laziness.
Some countries (like Australia and I think the UK) shorten mathematics to maths.
and other countries like the USA shorten mathematics to math.
I am Australian and I used to think that "math" sounded really weird. But, I am used to it now. :)
In Australia we are lazy I guess because although we spell it maths, we often pronounce it as mass.