Why do Americans simplify words?

They don't. Or at least they do it as much as the English do.

Language evolves. Our common language took two different paths when we set up a colony in America.

And using Z instead of S is not simplifying. In fact, under the language in England, using a Z is permissible. Frowned upon, but permissible.

Plus, most English people use the spellings you give for the Americans.

When the US was founded the language was still in a state of flux - not that many people wrote things down at all. As they developed and settled on spellings they did so independently giving rise to the (marginal) differences we have now.

Many authorities claim that the spellings are more logical.

So we have gray, being the color of greyhound buses.

Simple answer: Languages evolve.

More elaborate answer:

The three major differences between between American English and United kingdom (British) English are:

Pronunciation - differences in both vowel and consonants, as well as stress and intonation

Vocabulary - differences in nouns and verbs, especially phrasal verb usage

Spelling - differences are generally found in certain prefix and suffix forms

The most important rule of thumb is to try to be consistent in your usage. If you decide that you want to use American English spellings then be consistent in your spelling (i.e. The color of the orange is also its flavour - color is American spelling and flavour is British), this is of course not always easy - or possible. There just principal differences between these two varieties of English.

~David

I do not judge you so if you do not like it hang yourself buddy and that goes for everyone.