Why don't Function Lambdas Cast to Delegate?

[Q. Schroll via Digitalmars-d-learn]

Why don't lambdas cast to a delegate if they are of type R 
function(Args)? I don't see any reason to that; a lambda should 
be a delegate type by default, and a function only as a special 
guarantee/optimization. It just makes them cumbersome to use with 
toDelegate.
Probably there is a good reason why R function(Args) does not 
implicitly cast to R delegate(Args); I can imagine something 
internally (memory layout etc.) causes that. I'd just like to 
know.