It’s by explicit design. The problem is that while String most definitely implements IEnumerable<T>, most people don’t think of it, or more importantly use it, in that way.
String has a fairly small number of methods. Initially we did not filter extension methods off of String and the result was a lot of negative feedback. It almost tripled the number of methods at times with the right imports. With all of the extension methods displayed, people often couldn’t see the String method they were looking for in the noise.
String is a … simple type and it’s better to view it that way 🙂
It’s still completely possible to call an extension method on string. It’s just likely not going to show up in intellisense.
EDIT: String actually has quite a few methods. But because many of them are overloads they collapse in intellisense.