Why can't I explicitly return void from a method?

It's an interesting question. Since java enforces a return type (void is a return type) your first statement seems to make sense. I would take this only for convention. Since void is a placeholder and not an object, it was probably decided to leave it out for language coherency or compiler simplicity.

From JLS

A return statement with no Expression must be contained in the body of a method that is declared, using the keyword void, not to return any value (§8.4), or in the body of a constructor (§8.8).

further

To be precise, a return statement with no Expression always completes abruptly, the reason being a return with no value

A return statement with an expression returns the value of that expression. The type of cancel() is a void expression - it doesn't have a value.

Logically you want to execute cancel(), and then return - so that's what you have to say. The two actions (calling cancel() and then returning) are logically distinct.

Now Java could have a sort of "unit" type instead of void - but that would affect rather more than just return values.

It's like writing:

void v = (void) 1;
return (v);

So, I think void is not a type in Java. In C++, return cancel(); is legal. As a C++ programmer who is familiar with Java the answer is: Many things are not supported in Java syntax. Maybe for simplicity or readibility.

Note: A void f() declaration is similar to a procedure f() declaration in pascal and a procedure could not return any value such as functions, so we must call them in a separated statement.