Coming from the other side, it seems like "because you should be able to do it manually" seems like a poor reason to not provide the option to have things automated.

The computer is more accurate and faster at determining the type of an expression than humans are. That seems like a big part of the value proposition of static languages. So why not let it tell you the type in the editor instead of having to stop what you are doing to compile it (which may require fiddling with the code to get a meaningful type error)?