Why don't we require interfaces in dynamic languages?

The interface as a keyword and artifact was introduced by Java¹ ( and C# took it from there ) to describe what the contract an object must adhere was.

But, interface has always been a key part of Object Oriented Paradigm and basically it represents the methods an object has to respond. Java just enforces this mechanism to provide statically type checking.

So, dynamic ( OO ) programming languages do use interfaces, even thought they don't statically check them. Just like other data types, for instance in Ruby:

 @i = 1;

You don't have to declare i of type FixNum you just use it. Same goes for interfaces, they just flow. The trade-off is, you can't have a static check on that and failures are only show at runtime.

In the other hand Structural type ( or static duck type as I call it :P ) used by languages as Go or Scala, gives the best of both worlds.

_{1. See Daniel Earwicker comment about CORBA interface keyword}

We don't require them, but we do support them. Check out Zope Interfaces (which can be and are used outside of Zope).

Interfaces are used in statically typed languages to describe that two otherwise independent objects "implement the same behaviour". In dynamically typed languages one implicitly assumes that when two objects have a method with the same name/params it does the same thing, so interfaces are of no use.