In a C# event handler, why must the "sender" parameter be an object?

According to Microsoft event naming guidelines, the sender parameter in a C# event handler "is always of type object, even if it is possible to use a more specific type".

This leads to lots of event handling code like:

RepeaterItem item = sender as RepeaterItem;
if (item != null) { /* Do some stuff */ }

Why does the convention advise against declaring an event handler with a more specific type?

MyType
{
    public event MyEventHander MyEvent;
}

...

delegate void MyEventHander(MyType sender, MyEventArgs e);

Am I missing a gotcha?

_{For posterity: I agree with the general sentiment in the answers that the convention is to use object (and to pass data via the EventArgs) even when it is possible to use a more specific type, and in real-world programming it is important to follow the convention.}

Edit: bait for search: RSPEC-3906 rule "Event Handlers should have the correct signature"