Because StatefulWidget inherits Widget, which is marked as @immutable, any subclass of StatefulWidget must also be immutable (i.e. all fields final).
If you make a StatefulWidget subclass with non-final fields, it will result in this Dart analysis warning:
info: This class inherits from a class marked as @immutable, and
therefore should be immutable (all instance fields must be final).
(must_be_immutable at […] lib….dart:23)
And an explanation of how to use StatefulWidget from the StatefulWidget documentation:
StatefulWidget instances themselves are immutable and store their
mutable state either in separate State objects that are created by the
createState method, or in objects to which that State subscribes, for
example Stream or ChangeNotifier objects, to which references are
stored in final fields on the StatefulWidget itself.