Is there any reason why we have to explicitly declare static class members as static ?

The simple answer is "because that is what the language specification says".

The "real" answer I don;t know - but it's likely to be a combination of
1) It was never a compiler requirement that the class static attribute should be automatically applied to child members, so it just didn;t happen.
And
2) It means that when you look at a method, property or field, you can tell immediately that it is static - you don't have to look for the class header declaration to know.

For me, the second one is important: the class header could well be pages away, and if you forget...

And how much "precious typing time" does it save? Two keys? Three? Unless you are a very slow typist, that's very, very trivial - and lot less that the time you would waste looking for the class header!

Saving "typing time" is a poor reason for doing anything: people who do that are often the same ones who habitually use var instead of entering the type definition without considering how much harder it makes the code to read of others; and who leave all controls at the Visual Studio default names because that saves them time as well.