Why does Microsoft create specific Extension Methods?

I wrote my first Extension Method a few days ago.  Extension Methods are a great idea.

But I don't understand why Microsoft supplies specific Extension Methods.  The documentation for the String Class, for example, has many.  But if you are Microsoft why not supply regular Methods?  I used to avoid even looking at Extension Methods because I hadn't done any research and thus didn't understand what they were.  

Mostly I am just curious.  But I wonder too if maybe there is something I don't understand about Extension Methods which explains why Microsoft seems to (at least sometimes) prefer Extension Methods to regular Methods.

Thanks,  Bob