because we don’t know if it will be IEnumerable. A more real example would be like this:

dynamic x = SomeDynamicResultFromPythonOrSomething();
object o = x;
var result = from i in o where … select …;

The method which returns a dynamic may or may not return a dynamic which ultimately implements IEnumerable. Further more, depending on my implementation of Where, Select, etc. I might not even care if they implement IEnumerable. It may be enough that it returns an object which implements GetEnumerator. Remember that foreach does not require a type implement IEnumerable, just that the GetEnumerator method exist.