Plain old covariance and contravariance

[Reiner Pope]

I did some searching for variance here, and I couldn't find much 
discussion of it. My question (I think) is quite simple: why is 
covariance supported but not contravariance?

interface Foo
{
   Foo get();
   void set(Bar x);
}

interface Baz : Foo {}

class Bar : Foo
{
   Bar get() {} // Fine
   void set(Foo x) {} // Error, doesn't implement Foo.set
}

For some reason, Bar.set isn't taken to be the implementation of 
Foo.set, even though it will work for any parameters that Foo.set does.

What am I missing?

Cheers,

Reiner