...because it is American English as the default, of course. I tried to install NZ on mine, but it didn't work. British English (and it's 'descendants', New Zealand and Australian English) use an S. British English is also one of the, if not the, most popular English for foreigners to learn. After all, the language English was invented by... THE ENGLISH.
I'm from New Zealand. It's technically a piece of the British Empire, but we are considered independent, the Queen never does anything here, we have our own laws and government etc.
Keep in mind I'm not saying it is wrong to spell it with a Z (in America at least), but it isn't wrong to spell it with an S either, so there is little point in 'correcting' someone who does. That is all.
...because you're spell check is set from American. It's no big deal, really. I'm just saying that it isn't wrong to have an S. Same with customise, recognise, and many other Z words. They started with the S, actually. Americans just got lazy and said them with a Z sound, and then they changed the spelling to match the pronunciation.
There there are the U words: humour, neighbour, colour - American accents don't really pronounce the U anymore, so they eventually removed that too. They simplified entire words, like 'gaol' to 'jail' (though 'jail' has become common in other forms of English too, in the same way 'cookie' is being adopted into other versions of English by sheer presence).
English is a 'mongrel' language - it's made out of scraps borrowed from other languages, or 'bastardised' versions (words rewritten acordding to accents: "vamoose" (from the Spanish verb vamos, which means "Let's go") for example). It's constantly changing. It's becoming genuine spelling to use the word "thru" ("through") now (much to the opposition of linguists). Which in the end means that though "yoo kan stil spel stuf rong"...
...uhh... whatever. I mean, I understand you pointing it out, but several people had already said the same thing. And since had gotten the reply 'spelt with S in some places'. Just go with me on this. Spell check is country specific. Most English dictionaries will have alternative spellings of words (as in, will list both colour and color, though usually as the same listing).