I’ve been pondering about the C# and CIL type system today and I’ve started to wonder why static classes are considered classes. There are many ways in which they are not really
Your question is "why do I have to type the words static class X rather than foobar X". The answer is, because programmers already associate the word 'class' with 'a bundle of tightly packed encapsulated functionality someone wrote for me'. Which, coincidentally, fits perfectly with the definition of static classes.
They could've used namespaces instead, yes. That's what happens in C++. But the term 'static class' has an advantage here: it implies a smaller and much more tightly coupled group of functionality. For example, you can have a namespace called Qt or boost::asio but a static class called StringUtils or KWindowSystem (to borrow one from KDE).