There are a number of third party modules out there that do just this.

As to why its not in the core - hmmm - each MODULE in the core has a life of its own - and a lot of what is in a module gets driven by the person or persons who have put up their hands to manage that module.

Many modules have been templated, but the login and registration modules have never gone down this path.  Part of the complexity behind this issue with the login and registration modules is that these modules are actually provider based - that is - there can be any number of login systems installed in dnn - these include Active Directory, Facebook, Windows LIVE and of course the default DNN.

And as such the default DNN provider does not currently support templates or custom layout - if you want this there are 3rd party modules that add this functionality or you can look at coding your own provider.

Westa