Wikipedia describes the Single Responsibility Principle this way:
The Single Responsibility Principle states that every object should have a single responsibility, and that responsibility should be entirely encapsulated by the class. All its services should be narrowly aligned with that responsibility.
The traditional use of the controller in MVC seems to lead a programmer towards a violation of this principle. Take a simple guest book controller and view. The controller might have two methods/actions: 1) Index() and 2) Submit(). The Index() displays the form. The Submit() processes it. Do these two methods represent two distinct responsibilities? If so, how does Single Responsibility come in to play?
Yes it does.
And if you want to follow the SRP, you disaggregate your Controller into a Dispatcher and Actions; the Dispatcher dispatches control to its actions, and at compile-time (C++ templates) or at runtime (Java XML, whatever), you'd compose Dispatchers and Actions.
Why don't we see this more often? Because Controllers are often "ad hoc" implementations, leaf-level concrete classes that aren't generalized and aren't meant to be subclassed. Here, the class is used more to conveniently group code, the actions are almost certainly non-public (likely private, maybe protected), "merely" internal implementation details.
The choice of how to decide what action to dispatch to, the number and diversity of possible actions, is high, and dispatching and action are tightly-coupled. So in practice, it's often easier to just put the code together in one place.